@Lemmchen the different client-networks csn be the problem.
Some providers block webrtc or stun/turn…
Did you try to test your coturn on
webrtc.github.io/samples/src/c…
Delete all servers there and fill in your stun and turn (turn incl. credentials) andvtest ist.
You should get relay (=turn) and srflx (=stun) marked entries
@Lemmchen
This is only stun.
For a working turn you need also “relay” entries