I've just replaced my netgear nighthawk x6s, and my gateway is/was fine on my unit. That being said, the most recent firmware is currently broken and old (in true netgear fashion), so perhaps it is a problem if the router has a new firmware update.
I did upgrade, then downgrade the firmware before I boxed it up and replaced it, however but both versions are not new releases, by far.
Maybe push a pin in the reset button and see if it returns.
ipconfig /all (in a cmd prompt should show your current gateway)
Best of luck!