My Xbox LIve Nat type is strict?

well ive tried everything i have a belkin router, ive opened ports 88,80, 53 and 3074 in the right type(e.g tcp/udp) and ive even tried turning off firewall completely, and enabling and disabling upnp- nothing seems to work. then when i tried to put my xbox in dmz i couldnt even connect with a message saying my mtu needs to be at least 1364 when it is actually 1474 ANY HELP WOULD BE VERY APPRECIATED