Sky dont like you replacing their own router, so much so its a breach of their T&C’s.
Its a bit of a chew on to get the username and password off your sky router, and you then have to spoof the MAC address of your nighthawk. If you want to use it in place of the SKY one.
I tried the same thing when I was on sky, albeit standard broadband.
In the end I just disabled the wifi on the sky router and added my own router behind it in an AP configuration. Yes its an additional box, but it improved my wifi range and gave me Gigabit LAN speeds (internally only of course)