Before you do anything else, power down everything (including the Westell), and let it all come back online.
The Westell 7500 appears to be a "residential gateway".
That is to say, it's both a combined modem/router in one unit.
I don't have one, but I also have a residential gateway type unit with Frontier. It doesn't have fast wifi, and I wanted to use it with an (older) Airport Extreme.
The setup I chose was fast and easy.
I setup the Airport Extreme in "bridge mode", and left the residential gateway in charge of routing functions such as NAT.
Once the AE was up-and-running with wifi, I turned off the wifi on the residential gateway.
It's run fine like this for years now, nary a hiccup.
Might be worth a try in your case.