I do apologize for a late answer.
I have looked into the troubleshoot file, however, wasn't able to identify what could cause the issue. In the troubleshoot file the status shows connected.
Could you please try upgrading the firmware version to: RUT9XX_R_00.06.08.5 and check whether the issue reoccurs.
Firmware download link: https://wiki.teltonika-networks.com/view/RUT955_Firmware_Downloads
In addition, I will also send you our RUT9_R_00.07.01.1 firmware version. It has been publicly removed due to a severe bug with Ipsec VPN, however, if you are not planning to use Ipsec VPN this firmware could help you to get a more stable mobile(cellular connection). I will send it via private message.
If none of my provided firmwares would help, please download the troubleshoot file one more time. However, try to make sure that the file is download after the issue occurs as the logs might indicate what the underlying issue is.