Hi, the modem is not detected as it has an unexpected PCI vendor/device ID.
In [Custom build] GL.iNet GL-X3000: OpenWrt 25.12 with RM520N-GL 5G on PCIe/MHI I have prepared an image for the GL.INET x3000 that contains this modem, and all the pieces needed to rebuild it.
I hope this helps anybody willing to run this modem on vanilla.