I recently spent several weeks off-grid at communist permaculture farms in Tennessee, dozens of which orbit around places like Short Mountain. I talked at some length with people there about the need for a completely off-grid instant messaging solution which does not rely on the internet or any traditional infrastructure.
One of the ideas we came up with is what I called a “Cyberpony Express.” Basically the idea is that we set up a bunch of Raspberry Pi computers in ruggedized cases which can run off of solar power and connect to a software defined radio. This would allow people living at these communities to send simple end-to-end encrypted text messages across the region.
Here are some preliminary technical source documents I’ve found…
https://spectrum.ieee.org/amp/build-a-longdistance-data-network-using-ham-radio-2650417608
https://en.wikipedia.org/wiki/Packet_radio