What is the internet quality, where the RUT device operates?
Does the ZeroTier connection reestablish or does it require router's restart?
Is there any reason to use 6.09.2 and not the updated firmware?
Could you attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.
Files are private and visible only to Teltonika Moderators.