Hi @Daumantas and thank you for your reply.
So I have actually already followed the article in your first link and have previously set up a Guest WiFi network which was limited to the WAN interface. I successfully did this last year and it worked.
However, the key difference was that at that time, I was using the router’s 4G modem as my WAN connection, and therefore my internet connection was automatically part of the built-in WAN interface.
Recently I have stopped using 4G and I am connecting to the internet via my new ISP-supplied router which is connected to the RUTX12’s LAN1 port as per my diagram. Therefore, my RUTX12’s WAN interface is now Status: Down which means I can’t use it in firewall rules.
(Could I just plug my ISP router into the RUTX12’s WAN port and re-activate the WAN interface again? Yes, I could. However, there are actually other LAN devices attached to the ISP router as you can see from my diagram (e.g. “NAS”, “PC 1”). So presumably then my Guest WiFi would get access to those LAN devices which is undesirable… or those LAN devices would be totally inaccessible even from the RUTX12’s regular WiFi which is also undesirable. Please correct me if any of this is wrong.)
So I hope you can see my problem - if I understand it correctly, my RUTX12 is not actually aware that a WAN network exists - all it knows is that there is a LAN interface and it’s sending all traffic there.
What I am trying to achieve is to tell the RUTX12: “Hey, that connection in your LAN1 port actually exposes a WAN and a LAN. If any clients on Guest WiFi request a WAN connection, allow the traffic to go via LAN1. If they request a LAN connection, reject it.”
Or in “firewall pseudocode”:
IF [source_interface] is "Guest Wifi"
AND [destination_IP] is in any of these ranges:
[10.0.0.0 - 10.255.255.255]
[172.16.0.0 - 172.31.255.255]
[192.168.0.0 - 192.168.255.255]
THEN block the connection
ELSE allow the connection
I’m sure this must be possible - either by using VLANs or just plain old firewall rules or something - but I just don’t know enough to know how to set it up.