Nighthawk App for extender AC1200

[Apodasr]

I am stuck on the set up. 

I am using the APP through my Iphone

I am at the part where it is to connect to my network (Century Link)

and here is the problem

"Trouble connecting to myXXXXX WiFi network"....go to settings and coonect to Mywifi network, and return to the Nighhawk app. 

Try Again 

doesn't work. 

the ligths on the Century Link modem are all green

the lights on the Extender are all green

I can search the Web on the iPhone

I have turned off the cellular option on the iPhone

I have rebooted the Century Link modem

I have reset the Extender 

stuck!