Receiving NAT punchthrough attempt from target 2500 failed

Hi, I have set up a multiplayer game using Unity’s test master-server.

When my friend on another WiFi try to connect it says:

Receiving NAT punchthrough attempt from target ###########:25000 failed

How can this be solved?

EDIT: It works when I have 2 instances running on my computer.

Thanks, Andreas.

Don’t know if it is still a problem for you, but I think this might be because one of you does not have the correct port opened. Check if both of you opened the port 25000