I’m sorry for jumping in here but did you try downloading and installing the latest MODEM firmware? Not the ROUTER firmware. The modem firmware has profiles(MBN) for different cellular networks. If your router can not find a profile to connect to, it can reboot trying to switch SIMs or trying to select another profile. I would install the 4.3.0 Modem firmware, then follow this link to get the latest R03 firmware: GitHub - 4IceG/RM520N-GL: Quectel RM520N-GL is 5G IoT module specially optimized for IoT/eMBB applications.
Then follow the directions here to install the modem firmware: Constantly Loosing Cellular Connection on Spitz x3000 - #66 by makkie
Only after you have installed the newest modem firmware, then download and install the latest Router firmware: GL.iNet download center
Let us know how it works for you then.