RUTM51 LTE speeds are abnormally low

I’m having persistent issues with my RUTM51 as evidenced by my post history. While stability of the modem in particular has improved with firmware updates that I’ve previously coordinated with Teltonika support, the performance of the modem has degraded substantially from since before the updates. I’ve previously been able to achieve a downlink of 100+ Mbps, but am now stuck on at most 50 Mbps even with an external XPOL-24 antenna attached and excellent signal. That’s way below what’s expected of even a cat 12 LTE modem.

Here are the measurements of my router and phone:

Metric Router Phone
Cell ID 58136115 58136115
TAC 50203 50203
Band 3 3
RSRP -68 dBm -97 dBm
RSRQ -9 to -11 dB -15 dB
Downlink ~50 Mbps 127 Mbps

The RUTM51 is vastly underperforming and I’m at my wits’ ends as to what I can do to improve the situation. While doing speed tests the modem also crashed so stability is not yet solid either.