»
www.westell.com/images/d ··· uide.pdfThat is the Westell user guide for the CenturyLink version of the 7500.
Port forwarding starts on page 60, and allows you to specify which port range you're forwarding. I think this is what you're looking for, using the ports that HazyArc listed.
If that doesn't workI believe the SSI on page 120 is the equivalent of DMZ, perhaps you can set that to the XBOX's IP address and see if that fixes it?