Connecting Nighthawk RAX48 with Centurylink C3000Z

I recently purchased a RAX48 router to hook into my Centurylink C300Z. I had been using the Centurylink as a modem/router, but want to now use it only as a modem and the RAX48 as the router. The Nighthawk install app specifically says to not use bridge mode. I installed the RAX48, plugged it into the Centurylink. The wifi lights (5 GHz and 2.4 GHz) remain on in the Centurylink (as well as the RAX48). How do I know that the RAX48 is functioning as the router and not still the Centurylink? Do I somehow have to shut off the wifi in the Centurylink to cause it to function only as a modem? Again, since the Netgear app say not to use bridge mode, should I be shutting off "wireless radio" in the Centurylink? Thank you for any advice.