I had the NAT type issue on my Xbox 360 when I first got MW2. It was impossible to join any online game, and I was lucky to have someone on my friends list playing online because the only way to play was to join their game. Later on, it changed automatically but as far as I remember, there was a certain website that you have to go on and choose your router type, and it will change your NAT type, however I believe it specific for the application you are using.
I'm not sure because I didn't have the issue for too long, but you're gonna have to do some research based on what I said.
Good Luck!

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