Could you please clarify the following sentence: "I cannot get access to the WebUI of this device (but via LAN access to the UI is available)."
Are you able to access the WebUI when connected to the device via LAN? Perhaps you meant that you cannot connect to the WebUI remotely (which would be understandable since there is no internet connectivity).
Is it correct for me to assume that the issue is related to mobile connectivity?
To help diagnose the issue, please replicate this issue. Wait for ~5 minutes after rebooting to allow the device some time to attempt establishing mobile connectivity, and then download a troubleshoot file.
In case you are unable to access the WebUI when the issue is present, then you will need to save the logs in the flash memory. To save the logs in the flash memory, navigate to System -> Administration -> Troubleshoot and change logging settings to save logs in flash memory -> save & apply. Saving logs into flash memory ensures that the logs are retained even after the device is rebooted.
So, when logging is set to the flash memory and you experience the issue again, simply restart the device and download a troubleshoot file. Attach the file by editing your question. The attached files are only visible to Teltonika moderators. The troubleshoot file may provide insight into the issue you are experiencing.