Latest video asked for the best messenger app and recommended Signal, but that requires a real phone number to sign up. You could theoretically buy a burner with cash and then sign up, but Session lets you sign up without anything.
It’s also end to end encrypted and very similar to Signal. Hardly anyone is on it though, but if you want to private chat with a mate:
Can’t see how to add a link on this forum, so text it is.
This does not replace cellphone or internet! But if you look into LoRa protocol, it is very impressive; you can send data over many, many miles. The key here is line of sight and a strong mesh network at you location. Hardware required for this is dirt cheap.
You can have a microcontroller that handles LoRa traffic as well as the messaging. This project is a complete software solution.
The software is not mature yet, but this allow to private message over LoRa and other medium. Reticulum is a a network stack, but meshchat is something that you run on your desktop.