I need a messenger app that is working for over 100 meters without internet and i want it to be as stable as possible.
I do not know the range, but maybe look at Briar. It doesn’t just use the internet, it can also use Bluetooth (what Bridgefy uses) to send messages. It’s been around for quite a few years, and is arguably better than Signal, for privacy.
Afaik bluetooth and wifi aren’t reliable at 100 meters and won’t work at all at 200 meters. If you require a p2p connection you might be interested in the lora protocol. It has a range of 5-15 kilometers, but requires special hardware that most devices don’t have. There might be some microcontrollers with lora support, but I don’t know if any production apps are being developed for your use case.