Greetings,
Apologies for the delayed response.
Please have a look at the following topic and update the modem firmware according to the provided instructions: Known Issue With RG520NEBD Modem Firmware (Update Required)
If updating the modem firmware does not resolve the issue, please let me know.
And as a temporary fix, you can configure a Ping Reboot on the device. This will automatically reboot the device after a data disconnect, so manual reboots are no longer necessary. More information can be found here: RUTM52 Maintenance - Teltonika Networks Wiki
Best Regards,
Justinas