I have been doing tests with a borrowed modem and have now reconnected the original modem. Although I carefully checked the antenna connectors many times, it now seems that there may indeed be an issue with this, but there could also be a deeper fault with the modem. I still can't be 100% certain.
Can anyone explain to me what the practical significance is of differences in the stats? For example, if RSRP goes from -118 dBm to -102 dBm, what does that actually mean in terms of the modem's performance? How much of a signal strength difference does it represent? I don't really understand what RSRP, RSSI, and RSRQ are saying. All I can do is compare the differences and match that to test speeds, but there do not seem to be clear patterns in many cases.
RSRP dropping from -102 to -118 is not good and does tend to indicate antenna cable issue.