Hi there, hope this is not too much offtopic, but i created a OpenWRT package which is (for now) only tested and built for a banana pi r4 with a queltec rm520n modem. With this package you get a kernel sensor for the modem and can use it for pwm, hwmon and so on. Cause i also got some heat problems, i have installed a pwm fan above the modem and connected it to the gpio pwm. With this package i can regulate the fan by the modem temp 
Introducing the Quectel RM520N Thermal Management Tools, an OpenWRT package designed to manage the temperature of the Quectel RM520N modem. This package is in its early development stage and currently lacks a pre-built IPK file; users need to integrate it into their custom OpenWRT builds. A future release with an IPK file is planned. Key features include:
- OpenWRT Integration: Seamlessly integrates with OpenWRT build systems.
- Dynamic Device Tree Overlay (DTO): Supports dynamic registration of virtual sensors.
- Configurable Daemon: A userspace daemon reads modem temperature via AT commands and updates sysfs, virtual sensors, and hwmon nodes.
- Kernel Modules: Provides modules for sysfs-based temperature reporting, hwmon integration, and virtual thermal sensors.
Contributions and feedback are welcome to enhance this tool’s functionality.