check out https://www.meshtastic.org/ –
buy these direct , and already loaded off ebay – or roll your own with and ESP32 off the shelf .
communication without mobile coverage
the basic idea is you connect via bluetooth from an android phone app to the device and it communicates with the mesh network on a free band to other devices on the same channel – still a work in progress but looks amazingly promising for coms up to a few miles.
when using the gps modules they also give your location to other nodes allowing basic direction finding . great idea for remote hunting spots where cell phone coverage is bad.