I'm certainly no expert in this but what I can suggest you try is to enable DMZ to the IP address that is assigned to your XBox.
When your Xbox is connected and running a network connection, your router should give you a list of all the connected clients and also their IP addresses (192.168.x.x).
DMZ will likely be under the security section of the configuration page. You should be able to assign one local IP address (your XBox) to use one WAN IP (your internet connection) thorugh a DMZ (Demilitarized Zone,ie. unrestricted access).
Hope that does the job for you.
This thread is now locked and can not be replied to.