Can the AC1750 serve as an extender?

I'm trying to log into my AC1750 R6400 router so I can convert it into an access point. I'm doing this through the FIOS Modem/router that Frontier provided. Unfortunately, when I try to login to it through www.routerlogin.com or www.routerlogin.net, Netgear tells me:

You are not connected to your Router’s WiFi network. To access routerlogin.net, your device must be connected to your Router’s WiFi network. Check your current connection and try again.

I cannot get into the Netgear router to change it's settings. 

How can I bypass the Frontier modem/router? Even the entry to it is coax and mine is ethernet.

Thank you.