I have 2 APU4D4 's and have installed Quectel EC25 modems in mPCIe slot 2. I'm running Ubuntu server 20.04 and I've also tried DD-WRT. In both OS's the modem can't read the SIM card. I've tried both slots with no luck. I took the modem out and put it in a mPCIe to USB adapter that had a built in SIM card slot and everything worked perfectly, I was able to read the SIM and attach to the cellular network.
In looking at the schematics it appears that there is a SIM switch between the SIM sockets and the mPCIe slots. I wonder if that is the source of the problem?
Any thoughts on trouble shooting my 2 APU4D4 boards that act the same?