The Airport Extreme cannot connect to your Frontier router using only a wireless connection. It must connect using a permanent, wired Ethernet cable connection if you want the AirPort Extreme wireless to have Internet access.
It is possible to set up the AirPort Extreme to provide a wireless signal that will not be able to connect to the Internet, but, in order to back up, you will have to manually log off of the Frontier network and then log onto the Airport Extreme network to back up....then, log back onto the Frontier network when you want to have Internet access again.
My recommendation would be to set up the AirPort Extreme with Internet access and then use the AirPort Extreme wireless network to back up to the USB drive at the AIrPort Extreme. Backups will at least occur faster this way, because the AirPort Extreme will have faster wireless than the Frontier. You could use the Frontier wireless for guests, or the kids, or if you don't need another wireless network, there is likely a way to turn off the WiFi function of the Frontier router.
Another advantage to using the AirPort wireless is that backups will occur automatically, and you won't have to manually switch networks to back up and then switch back to another network for Internet access.
Frankly speaking, if your goal is more efficient Time Machine backups, an AirPort Time Capsule would be able to back up 2-3 times faster than the AirPort Extreme with an attached USB drive.
An even better option would be to simply connect the USB drive directly to your Mac and back up that way. Backups will be more reliable this way and faster as well.
But, it's your call.