[sudo-discuss] need a cellular modem or android help

Hi community, I'm having to do a job where I install a linux machine on a vehicle, and I need to be able to connect to it over the cellular network. I have a Google Fi data SIM, which should work with most modern cellular devices that are unlocked. I have a Sierra Wireless MC7455 miniPCIe card and a USB adaptor, but it restarts every 43 seconds and I can't tell whether it's a hardware or software problem. I'm going to keep trying it, but it might be a dead end. I have a Nexus 6P cellphone, which is rooted, and which definitely works with my SIM. The screen is broken but i have a root login over adb, and I have an ssh client from an app i installed before the screen broke, but it doesn't give me a root login and su doesn't work from ssh (although i will keep trying) but with the android, I have an uphill battle because i have no idea how to curate the various processes that will take up data bandwidth, and since i'm paying per data for the SIM, i want to make sure that only my own important data gets sent through the cellular network. I want to install wireguard, which I understand might be a challenge, although I can probably just install wireguard on the downstream beaglebone black which will be running regular debian. If anyone wants to help me poke at the android operating system from root ssh (remotely by phone and screen-sharing of a session like tmux or screen) that would be greatly appreciated. I am also open to buying hardware if anyone has a USB cellular modem that you think is unlocked and will work out of the box with a Fi SIM and especially if you know how to get it to work from linux (something i've never done) Thanks everyone! -jake