Any modern router can do open NAT. You just need to forward the right ports.
http://support.xbox....nat-type-strict
I have those ports forwarded to my XBOX and MW3 says I have open NAT.
My NAT should be open as I have forwarded the right ports, at the moment the XBOX is a DMZ instead which should have the same effect but still Strict Nat.
It's because I have a two router setup, a modem and a wireless router. I will buy a wireless gateway which will eliminate the issue. My modem model is known for bad NAT
Can you forward the ports on your modem to your router? I had to do this on mine to support web administration for a pc app I use.