Greetings,
While the issue is present & during normal, and operational times, what signal strengths are you getting? These can be viewed in the device WebUI, under Status → Network → Mobile: RUTM51 Network - Teltonika Networks Wiki
If you’re getting “Poor” signal strengths, speed drops & disconnections are very much bound to happen. The reasons for this can vary depending on the use case, environment, antennas, provider & configuration. I would suggest looking into Band Locking as an option to try locking onto specific bands (just go through them 1 by 1 to see if one of them gives you good results): Band Lock - Teltonika Networks Wiki
Other than that, as mentioned, it is also use case & location dependent, so could you possibly clarify these conditions for your device as well?
As you’ve mentioned this as well, could you kindly tell me what exact firmware version you upgraded to?
There are also some suggestions mentioned by other users right in this forum thread, which could give you some useful insights or perhaps even resolve your issue completely: RUTX50 IPv4 connectivity started to fail & DNS resolution started to fail
Kind regards,
M.