RM520N-GL Docs, Files, and More
Various Info and Tutorials
Updating Quectel Modems in openWRT with CLI and a USB Drive
Make note: The latest modems have HUGE update files and generally there is not enough space on the router to update via copying the files onto the routers file system. If you are using your modem via USB tether, you will need to use a powered USB hub to have both on the USB bus on the router at the same time.
Here is one that we use regularly, including for the tutorial above: https://amzn.to/40Pxt3s