Hi there Cat0234,
What you are experiencing is common amongst most Router users, you cannot join a game where another user has the same problem as you - regarding NAT. Think of this in terms of magnetism, two likes cannot face each other, they repel.
You can get around this issue perhaps:
- by making your computer a DMZ host/server (having all ports forwarded to yourself)
- by using uPnP in your router settings
- by assuring that all your firewalls (both software and hardware) are disabled fully.
If you require any further, assistance, it'd be much appreciated if you could provide your router make and model number, together with the method you are attempting.