Naven
Answers
Can you send messages without an internet connection?
Not through an internet messenger. SMS works over the cellular network without data, and a few apps use Bluetooth mesh for short range, but anything that reaches a server needs a connection.
A messenger with an offline mode is generally queueing: it accepts what you type and sends it when a connection returns. That is useful on a patchy train and not a substitute for coverage.
SMS remains the fallback that works with signal but no data, at the cost of no encryption and a per-message charge in some places.
Naven queues rather than losing what you typed, and sends when the connection comes back.