Funnily enough now it would be quite easier to make a hidden one. Slap a GSM modem on one and just hide it anywhere where there is power source and you're essentially done and very hard to track.
Very easy to track... radio direction finding for a static source, even operating at low power output, is a solved problem. Adding a GSM uplink provides zero annonymity because you still need to connect via IP, which can ultimately be unpicked even with TOR or VPN masking if the infrastructure has been compromised.
Right, if you get FBI on your case maybe.
The GSM modem will need a SIM card with your number on it to work. Unless you can obtain an anonymous one, but I believe that is almost impossible in most countries.
SIM swapping and spoofing is a thing.
Its usually trivial to obtain a sim card anonymously even in countries with mandatory registration and ID laws (eg: certain European countries)