While I don't mean to take away from what the devs are doing (as it is awesome!), the modem is in two parts: a "user space" that is an version of Android (yes, the modem runs Android, I think Android 7?), and the actual modem has a lower level firmware based on Qualcomm Hexagon, which is still closed source, and not easily replaceable.

this seems to contain the latest status and build instructions: https://github.com/biktorgj/pinephone_modem_sdk

there are also some interesting reverse engineering details here: https://xnux.eu/devices/feature/modem-pp-reveng.html