cellular modems
You can add a celluar modem to a low power computer and interact with cell networks for things like machine-to-machine M2M communications, remote reporting, etc.
You have to figure out what kind of network you’re going to be talking with, like a GSM (super common worldwide), CDMA or whatever. Then you have to find a modem that will work with that technology and whichever carrier you want to talk to. GSM for Sprint is different than AT&T though they both still use GSM, go figure.
References:
modem seller:
https://techship.com/products/category/cellular-modules/
https://www.sparkfun.com/search/results?term=3G+shield
https://www.cooking-hacks.com/documentation/tutorials/3g-gps-shield-arduino-raspberry-pi-tutorial/
https://www.aliexpress.com/item/Free-Shipping-1pcx-GSM-SIM900A-GPRS-module-development-board-learning-board/1211480587.html
http://www.sainsmart.com/raspberry-pi-sim900-gsm-gprs-function-module-adapter.html?gclid=CPjWqda-7c4CFZGBfgodW0YEpg
http://uk.rs-online.com/web/generalDisplay.html?id=new