Factorio cannot establish connection to server

[LockKey]

Hello, I'm sure it been asked before but my friends and I aren't able to figure it out. We have looked online and read the tutorials but it just won't let us connect. However, for one of my friends it appears perfectly fine, he can invite us to his game, but we get the cannot establish a connection to server message, but he is able to join random games. But when my other friend and I host, we can't even invite people. Also, I was able to join a random public server, don't know if that's relevant