Hello,
To catch the reason as to why this is happening, I would recommend turning on logging to flash, so that logs are not lost after reboot, then after the router reboots, download a troubleshoot file and then attach it to the post, the file will only be visible to teltonika engineers.
To change the log saving from RAM to FLASH go to System -> Administration -> Troubleshoot. You will also be able to download the troubleshoot file from here.
Make sure to not leave the save log option as to save in FLASH as this will degrade the flash memory of the device