Update, because I am making progress:
The modem is now working if I follow these instructions:
Additionally, I can use minicom to connect to the modem on ttyAMA0 and send AT commands & get a response. Super!
Therefore I know that the hardware is working, and the modem is up (no additional GPIO switching is needed).
I would however like the modem to be recognised & controlled by modem manager, from the hostOS (so that connectivity is not controlled from the container, for obvious reasons).
Therefore, is there a way for me to see why modemmanager is not recognising this modem? I am sure it is supported, because if I use the USB cable it is recognised instantly.
(PS: some other forum said I needed to set the following udev rule, which I did. Too bad this didn’t help:)