Xbox NAT is set to Strict or Moderate why playing online.

Yes you can leave uPnP enabled and you don't need to set up any Port Forwarding or Triggering. 

You should be getting OPEN NAT if you connect just 1 xbox to the modem. How mine works. 

FYI, Netgear has set up a community forum specifically for the Cable Modem products. Might be more likely to find someone who has a solution if the question is posted there:
https://community.netgear.com/t5/Cable-Modems-Routers/bd-p/home-cable-modems-routers

Please use this link to the main forum product list to review and choose where to make your posts. 
https://community.netgear.com/t5/NETGEAR-Forum/ct-p/en-netgear

Thank you.