"A bulletin board that works on your phone" is actually a decent summary of gossip protocol based p2p systems like Secure Scuttlebutt (
https://scuttlebutt.nz). Data can be transferred when devices are in near enough proximity to each other (share a network, bluetooth, whatever). Messages are cryptographically signed (and possibly encrypted) so you could use this as a way of maintaining (albeit slowly) communications across a distance so long as there is still travel between communities.