Intermittent extremely slow websites & downloads on 5G (RUTX) – full speed with VPN enabled

Hi everyone,

I’m experiencing a persistent and confusing issue with a Teltonika RUTX50 router on a 5G connection.

Setup

  • 5G connection with excellent signal quality

  • Speed tests consistently show 700–900 Mbps down and 100 Mbps Up

  • Issue affects multiple devices (Windows, macOS, mobile)

  • Same behavior via LAN and Wi-Fi

Symptoms

  • Some software downloads (.exe / .zip, CDN-hosted files) are limited to ~300–500 KB/s

  • Some websites load extremely slowly:

    • Images load line-by-line or stall

    • Pages partially load or time out

    • Even large sites like Amazon are affected at times

  • Behavior is random:

    • Some days everything works perfectly

    • Other days browsing and downloads are painfully slow

  • Speed tests always report full bandwidth, even while pages crawl

Key observation
The moment a VPN (NordVPN) is enabled on the client:

  • All downloads immediately run at full speed

  • Slow websites load instantly

  • No other changes required

This strongly suggests the issue is routing-related rather than bandwidth-related.

What this points to
Based on testing, this seems related to one or more of the following:

  • ISP routing / peering instability (time-dependent, explains “good days / bad days”)

  • IPv6 does not work so both connections are running on IPv4

  • MTU / MSS mismatch on 5G + CGNAT

  • Traffic shaping or DPI affecting specific flows or destinations

The randomness over time suggests changing upstream routes or congested peering links rather than a local signal issue.

Questions

  1. Are there recommended MTU / MSS values for RUTX on 5G to avoid TCP performance issues?

  2. Is it known that IPv6 can cause intermittent slow browsing on some mobile networks?

  3. Can IPv6 be disabled or deprioritized on the WAN interface?

  4. Has anyone successfully solved similar issues via policy-based routing or WAN tuning?

  5. Any RUTX diagnostics (tcpdump, MSS clamping, WAN stats) you recommend to confirm this?

I’d like to resolve this at the router level rather than relying on a VPN permanently.

Thanks in advance for any insights.

Hello,

What is the value of the MTU as reported by ifconfig ?

What is the output of:

gsmctl -A ‘AT+QMTUINFO’

Regards,

Hello,

If you could - please do answer vogon’s question as well, as the information would be valuable.

Usually, you would leave the values as they are, but you can try changing the MTU to 1420(increments of 20), for example, not recommended to go lower than 1280, as it may break connections even more.

No, but IPv6 isn’t widely supported yet - I would recommend forcing the PDP type on your mob1s1a1 interface (I presume you use mobile) to use IPv4. To do that, you would head to Network → WAN → “Edit” on the mob1s1a1 interface → PDP Type: IPv4 → Save & Apply

By default, there are 2 interfaces for the WAN, specifically, which are WAN and WAN6; one is responsible for IPv4, the other for IPv6 - you can disable the WAN6 interface.

Could you please confirm what firmware version is in use currently? This can be seen in the top right corner of the WebUI, or under Status → System - if you don’t have the latest, please upgrade. Next, please clarify - how often does this happen exactly? More specifically, does it happen, say, every 2 hours, or happens completely randomly? Do you have any kind of firewall/traffic rule-related changes made on the device? What power supply are you using with your RUTC50?

Regards,
M.

root@RUTX50:~# ifconfig

br-lan Link encap:Ethernet HWaddr

      inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0

      inet6 addr:  Scope:Link

      inet6 addr:  Scope:Global

      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

      RX packets:25764310 errors:0 dropped:1847546 overruns:0 frame:0

      TX packets:14220756 errors:0 dropped:0 overruns:0 carrier:0

      collisions:0 txqueuelen:1000 

      RX bytes:6135379652 (5.7 GiB)  TX bytes:3803911777 (3.5 GiB)

eth0 Link encap:Ethernet HWaddr

      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

      RX packets:177931630 errors:0 dropped:3902 overruns:0 frame:0

      TX packets:340631452 errors:0 dropped:0 overruns:0 carrier:0

      collisions:0 txqueuelen:1000 

      RX bytes:105422650826 (98.1 GiB)  TX bytes:488988527599 (455.4 GiB)

eth1 Link encap:Ethernet HWaddr

      UP BROADCAST MULTICAST  MTU:1500  Metric:1                                                                                  

      RX packets:0 errors:0 dropped:0 overruns:0 frame:0                                                                          

      TX packets:0 errors:0 dropped:0 overruns:0 carrier:0                                                                        

      collisions:0 txqueuelen:1000                                                                                                

      RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)                                                                                      

lo Link encap:Local Loopback

      inet addr:127.0.0.1  Mask:255.0.0.0                                                                                         

      inet6 addr: ::1/128 Scope:Host                                                                                              

      UP LOOPBACK RUNNING  MTU:65536  Metric:1                                                                                    

      RX packets:89824 errors:0 dropped:0 overruns:0 frame:0                                                                      

      TX packets:89824 errors:0 dropped:0 overruns:0 carrier:0                                                                    

      collisions:0 txqueuelen:1000                                                                                                

      RX bytes:8611817 (8.2 MiB)  TX bytes:8611817 (8.2 MiB)                                                                      

qmimux0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00

      inet addr:P-t-P:10.84.57.39  Mask:255.255.255.255                                                              

      inet6 addr:  Scope:Link                                                                         

      UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1                                                                  

      RX packets:32232380 errors:0 dropped:0 overruns:0 frame:0                                                                   

      TX packets:10801929 errors:0 dropped:0 overruns:0 carrier:0                                                                 

      collisions:0 txqueuelen:1000                                                                                                

      RX bytes:38895167427 (36.2 GiB)  TX bytes:8581518609 (7.9 GiB)                                                              

wwan0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00

      inet6 addr:  Scope:Link                                                                         

      UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:31744  Metric:1                                                                 

      RX packets:2 errors:0 dropped:4888 overruns:0 frame:0                                                                       

      TX packets:97592756 errors:0 dropped:0 overruns:0 carrier:0                                                                 

      collisions:0 txqueuelen:1000                                                                                                

      RX bytes:288 (288.0 B)  TX bytes:99243174360 (92.4 GiB)                                                                     

root@RUTX50:~# gsmctl -A ‘AT+QMTUINFO’

+QMTUINFO: 1,1500,-

+QMTUINFO: 2,1500,1500

Hi,

Edited your message to remove the sensitive details (SIM card IP & Mac addresses).

For the command output - thank you, looks like all is well here. As for my other questions, I’ll be waiting for answers on that end.

Regards,
M.

1 Like

Thank you so much for this excellent support. I compiled the report with some more information. Is there any way we can make this into a private chat?

Hi there, pxs,

We can move to a different channel, of course. I’ve sent you the form, which you’ll receive in your email inbox. Simply fill it out with your contact details, and in the TICKET ID field, fill out 17452, which is your Thread ID.

Regards,
M.

From br-lan:

And from eth0:

Strange. The br-lan interface has a 7% RX drop rate but the eth0’s one is small.

Do you use the wifi interfaces ?

How are you able to see the drop rate? I’m not using the Wi-Fi interfaces on the router itself, but the router is connecting to repeaters (DECO) inside the house. Router is set as NAT and repeater on AP mode.

From ifconfig br-lan. 1847546 / 25764310

Could you draw a diagram of your network ? A handwritten one will do?

I submitted the ticket, but I am not able to answer directly to your emails. How can I communicate with you outside of this thread?

Hello,

Once the form is filled out, a ticket gets created in a separate system. Afterwards, we need to reply to the said ticket for you to receive an e-mail, to which you can then reply. If you haven’t gotten anything, means I haven’t replied yet.

Regards,
M.