Dio Brendo

We recently picked up the game and started trying to get multiplayer going. I've hosted a dedicated server, all the port forwarding is working properly, both the main game and server are allowed through our firewalls, and one of our friends can connect just fine. For two of us however, no matter who is the host the other one can't connect. We both get an error saying "The connected party did not properly respond...". I've been digging for information and seeing other people with the same error but none of the fixes posted have worked so far. For clarification, there's 3 of us playing and one person can connect to both others no problem, but the other 2 can't connect to each other at all.