I use two devices. The first runs a kernel+userland I can edit and acts as the gateway/AP and DNS server for the second, which runs some commercially-motivated, "locked-down" consumer OS.
I've been thinking about doing this and scrapping all but one of my data plans, and having a robust default-deny whitelist of allowed IPs/netblocks/hostnames on the phone vlan/ssid, but haven't worked out all the details yet.
How are you doing it?
Regarding LTE modems, I do not use a data plan on "locked-down" mobile devices for personal use. Somehow I have been able to survive on WiFi alone.
I’m looking at something like a raspberry pi zero, using the built in wifi to serve as an AP, powered from a large-ish USB battery pack, something that could run 18h+, with a USB LTE modem. Ideally I could get it small enough to strap to an ankle or something so I don’t need to bring a bag.