You can check the topics of fillipone and finaleshot2016, and get some ideas for off-grid transactions.

[Total privacy Bitcoin]: off grid Transactions LoRaWan/goTenna[url=http://[url=https://bitcointalk.org/index.php?topic=5226051.0]Bitcoin Transaction without Internet.]
[url=https://bitcointalk.org/index.php?topic=5226051.0]Bitcoin Transaction without Internet.
A LoraWan TTN compatible firmware for a cheap ESP32/Lora board. (Github).