Check LoRaWan and meshtastic client. 20USD off the shelf equipment give you a descentralized mesh network.
Discussion
Need it standardized and available in every phone.
You can connect to these devices via BLE (and Wi-Fi) and it will extend the mesh by RF signal to the next device and so on. The device will work as gateways and will retransmit what ever message (encrypted) they receive, similar to torrent files. Meshtastic is open source, building the add-ons to connect to other protocols like nostr, lighting, etc. should be no friction at all.