The device runs a custom version of OpenWRT that is easily replaced with a standard release

Running at only $29.99, this low power little router is easily hacked and useful as a lightweight Linux-based SBC, especially for simple MIPS work.

https://blog.jmdawson.co.uk/the-gl-mt300n-a-20-hackable-linux-router/

https://www.amazon.com/GL-iNET-GL-MT300N-V2-Repeater-300Mbps-Performance/dp/B073TSK26W/

the modem runs its own OS totally separate from the rest of the PinePhone OS

While these can be acquired in many devices, for many purposes, it appears that the easiest way to access the underlying Linux firmware is with a PinePhone. Capable of running basic things.

https://www.quectel.com/product/lte-eg25-g/

https://nns.ee/blog/2021/04/01/modem-blog.html