First, could you update to the latest version with Keep settings option disabled?
Next, have you tried to disable Auto APN in Network -> Interfaces -> Your_mobile_interface and enter APN value manually?
I would also like you to attach a troubleshoot file from the latest firmware to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there. Information in the logs will provide more details about your issue.
Files are private and visible only to Teltonika Moderators.