Ok, assign your XBox a static IP as per my last post. It needs to be static so the firewall rules always apply correctly.
Then on the router, open the following ports to the LAN IP you assigned the XBox..
Port 3074
Port 88
Port 53
Once you've done this, all should be well.
Edit: And had i bothered to click the link Hedge posted above i'd have known he'd already posted this lol