AT&T wireless homebase MF279

I have the wireless internet homebase mf279.  I’m trying to set it up to have an open NAT on my Xbox.  I have it hooked up straight to my Xbox through the Ethernet cord provided.  I have tried port forwarding, enabling uPnP, and also tried putting my Xbox IP address into the dmz mode on my routers settings.  NONE of these options have worked.  What can I do to get my NAT type to be open?  All I use the internet for is to play Xbox multiplayer games and do not need a firewall!  Please help , this is the only internet I can get because I live in a rural area.