Hello,
A brief suggestion would be to try using a custom APN, based on your service provider in Network -> Interfaces -> [mob1s1a1] settings -> [Disable Auto APN slider button].
Otherwise, I would like you to attach a troubleshoot file to your question. Please, make sure to replicate the issue with the new firmware version, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.
Attached files are private and visible only to Teltonika Moderators.
Best regards,