Plug in an EG25-G USB module. It has good driver suppport on linux as this is the same modem that's in the pinephone. About $60 plus the antennas. It also gives you GPS.

I'm actually working on this right now. I made a case for the module + antennas if you want.

EDIT: oops I hadn't read the article yet LOL. But the wisdom about the USB module still applies. There are also adapters for mpcie or m.2 or whatever modules to convert them to USB modules. Also you probably want to have a module or adapter with a sim card tray.

I bought mine off of aliexpress and also bought the LTE antennas. You need a separate antenna for GPS as well.

Lastly, with some of these adapters, the sim card goes in backwards relative to how you normally think it goes in. This had me stuck for a couple hours.