But your post didn't reveal any tech details so I'll try one time and hope this helps.
In your router make sure your 360 is in the DMZ.
Recently I decided to take my Xbox gaming from my home in Michigan to my cottage in Canada. The internet isn't great here (Xplorenet...basically runs one DSL connection and then uses cell towers to send it out to people in rural areas, like where my cottage is), but it works fine for web surfing and World of Warcraft.
Anyways, I haven't really had a chance to test it with Xbox Live as all I can do on Live is send/receive messages and watch video content on marketplace. I connect to Xbox Live just fine...I just can't join parties or games, or connect to EA's BC2 servers/Bungie's Halo 3 servers, etc. (people also couldn't connect to my party/games). I was told it was an issue of ports, so I bought a router that the guy in the store set-up for me himself (in-store), supposedly ensuring it's compatibility with XBL. Still no luck. I fiddled with the IP, the DNS, all that. Still no multiplayer games or party chat. All I can do is hangout on the dashboard (connected to live) and send friends messages.
My setup at home works fine (wireless), but here, even with the router, all I can do is hangout on the dash. I wondered if it was my NAT type but the Xbox test didn't show any sign of that. Any help would be much appreciated.