Most messaging services these days use your cell service or the Internet. But if you find yourself in a place with fluctuating signals and no Internet, then an alternative you can consider is TinCan. It’s an app that lets you send messages to other TinCan users who are within WiFi range, which is about 100 feet.
So how does it work? Well, the app appears as a WiFi hotspot with the ssid TinCan_(randomID) to users who don’t have the app. When the app detects someone with TinCan nearby, it automatically exchanges messages that they don’t have in common. That said, the messages are public, with its developer describing it as being “akin to Twitter or IRC.”