WebRTC does not work for me

I’ve been trying to get WebRTC to work with two Android devices for two weeks now and I can’t find my mistake, maybe I have a thinking error. As soon as I want to connect to another device with one device, only the one device gets an answer and a stream, but this one is empty, so RTCView remains empty.

Maybe I didn’t understand WebRTC properly.

Client:

Server:

Im sorry, I was just stupid.

The code above is a working example with server and the newest version