I just bought a new Netgear Nighthawk CM1200 to accommodate the larger bandwidth I'm paying for, but the Deco X20 does not play nice with the new modem. The X20 was working perfectly fine with my older modem, Arris Surfboard SB6121. When I swapped out the new modem I get no internet. I called the ISP and they said the new modem is registered and the line is connected. I called Netgear and they had me go direct to the modem to my laptop, instead of going modem to router then connecting over Wi-Fi and that worked. 

I've tried multiple configurations, including turning IPv6 on and configuring a static IP address for IPv4 and IPv6 and a custom DNS. Nothing seems to work. So bizarre that when I go direct to the modem, I have internet. I tried plugging the X20 to a network switch, which is directly connected to the modem, nothing.

I've also tried to put the router in AP mode, but that didn't work either.

Anyone have any ideas?