Meshtastic is a cool off-grid communication network. It is open source and doesn't require any cell towers or internet connection. As long as you have a device with bluetooth compatiblity, you can send messages via a meshtastic node. Every node in range can be used to "hop" an encrypted message (text only) from the sender to the recipient within 7 hops. There are a various number of developer boards available for use with meshtastic firmware and premade devices if youre not really into tinkering.