Devices that route connections on a network, or receive cable input. These will run a variety of architectures, although MIPS and ARM are very common. Operating systems will range from custom Linux-based OSes, to more conventional OpenWRT, DD-WRT, etc. variants.

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

It is recommended to backup the eMMC before flashing

A surprisingly high amount of postmarketOS' functions work correctly on these little Chinese(?) LTE dongles. SSH, WiFi, mainline, GPS, and more! Very detailed article documenting installing postmarketOS on these.

https://wiki.postmarketos.org/wiki/Zhihe_series_LTE_dongles_(generic-zhihe)