[ignoramous]

A YC company is (was?) working on creating mesh communication network from just mobile devices (not sure if they can be deployed right away independent of Internet connectivity): https://volkfi.com/

There are companies that sell software-defined radio products to help deploy volunteer-run radio networks, but those may be easy to jam and/or illegal: https://limemicro.com/products/boards/limesdr/

Not sure how far srsLTE (https://www.srslte.com/) can go here without requiring special equipment and Internet connectivity (someone well-versed can perhaps comment on its feasibility as a standalone backhaul): https://news.ycombinator.com/item?id=18569961

Walkie-talkies work nicely for comms but not secure, should work well enough for coded messages; however.

I stumbled upon now-defunct https://opentech.fund backed https://www.qaul.net a few days back, developed in response to censorship in MENA, that I personally like: They maintain a mapping of devices (similar to BitTorrent) over Wifi P2P and/or Bluetooth to create a decentralised secure routing network. No extra hardware needed.