Hello,
From what you are describing, it appears to be a hardware related issue.
In these instances, the simplest solution would be to return the device to your reseller for a warranty repair or replacement.
Otherwise, I would like you to attach a troubleshoot file to your question. Please, make sure to replicate the issue, 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.
Also, after the issue occurs, could you check if you get any output from the command below, by logging to router via SSH:
Best regards,