Few Guidelines to setup router or extender:
1. Make sure your device is plugged in
2. Connect Ethernet cable between router and modem.
3. Restart your modem as well
4. Connect an Ethernet cable to computer as well.
5. Now it's ready to set up.
Nighthawk ext setup app help you to setup different models of dual band, tri band netgear nighthawk routers and extenders.The models are like below

AC1750
AC2200
N300
N600
AC750
AC1200
Nighthawk X4 AC2200
Nighthawk X6 – AC3200
Nighthawk X4S DOCSIS
Nighthawk AC1900
Nighthawk X4S – AC2600
Nighthawk X6 R8000-100PAS
Nighthawk Ac2300
Nighthawk X6S AC4000