Xbox 360 opening my NAT please any tips welcome...?

I have a westell modem, a belkin router, and an xbox 360. My problem is my nat is closed. I have tried port forwarding all the ports xbl says to. still closed. i have tried putting my xbox in dmz which i never wanted to do in the first place, but it was my last resort. says i need to adjust the mtu. i cant find that function on my router. Any tips? Do i need to port forward the ports from my modem to the router and then from my router to my 360? i am able to port forward on both modem and router.