Xbox Live via Thomson Gateway TG585 issue

[#1]

Hi all,
I can play online ok but I'm having issues joining a party with friends. This seems to be due to have a closed NAT type. I've logged into the router and added the xbox live via the 'gaming service' option - this assigns UPnP to the xbox, but it's not doing the trick.
I understand it's worth trying to place the xbox in the DMZ, so to speak. With the Thomson router this is done by selecting:
Home > Home Network > Devices > Assign Public IP
It then gives the option to choose which device on the network to assign the public IP address to. Which would be fine, except my xbox doesn't appear in the drop down list
Any suggestions?  Thanks in advance

[#2]

Hmmm I am sure it came down on mine. May be a silly question on my part, but have you tried it with your Xbox switched on? Also your Xbox needs to be a static IP address for it to work.

[#3]

Thanks for the reply Steven
The xbox already had a static IP set up to get it working in the first place, so I don't know what else to try. Quite frustrating - can't play with my friends and I keep getting booted out of matchmaking searches and having them start again so I'm often waiting five minutes or so between games
Anyone else any ideas? I can add my Mac, my laptop or my phone to the DMZ but not the xbox!!

[#4]

hi all,
plz help me with this
same problem but i have no idea what to do!?!?