What Firmware is currently loaded? What is the Mfr and model# of the ISP modem the NG router is connected too?
You can however it's not needed. You only need uPnP enabled on the XR router and connect the xbox to the router via wifi or wired. Wired is preferred. Disable Instant ON on the xbox as well. If the xbox isn't fully connecting, press the front power button on the xbox until it shuts off. Then turn it back on. This will clear out any bad networking cached configurations.
As above UPnP should allow your Xbox to get an open NAT in the console network settings. I would advise you read this guide on NAT first but to answer your question you would put the Xbox IP as shown by the device manager in the DMZ in WAN Settings but this shouldn't be needed unless you've tried all other steps and they don't work. http://support.netduma.com/en/support/solutions/articles/16000076587-dumaos-optimal-settings-guide-o...
