Bridge Mode

I need help asap with my modem, it is an AT&T Modem, the model is MF279. I just bought a Nighthawk XR700 Gaming Router and my kid's Xbox is showing that the NAT Type is strict and there is a double NAT. I looked at some articles and it says to put the modem in bridge mode, but I can't see that option in the settings of my modem. If you could help me I would greatly appreciate it. Thanks