How to communicate on trips with limited internet
Trails, roads and campsites: what a group in the same place can still do when carrier signal fails.
Direct answer
If the group is on the same stretch of trail or the same campsite, an Android proximity app can keep a local thread even without 4G. Anyone who stayed in the city still depends on the ordinary internet.
Before you leave
Install KingFy, open offChat and test Bluetooth permissions (and Wi-Fi Direct if it exists) at home. On the trail it is too late to learn that a permission was denied.
Agree that everyone uses the same app. offChat does not deliver to someone who only has SMS or another messenger.
What not to do
Do not treat offChat as an official emergency plan. Keep the trail procedure, tell someone the route, and do not replace professional safety radio when the activity requires it.