Connection failed

Hi everyone, we are making a game with coop option. We tried to connect to each other through different wifi from different houses. It worked a bit before with a bit of lag. Now it doesn’t anymore

Do you know what can possibly cause this? A list of possibilities would be nice
Any what can we do to see where we can find the problems?

Thanks in advance

Come on guys. We have some million Unity active users and not an answer?