Xfi Gateway Modem Update

I have the Xfinity Advanced Gateway (XB6), which I've had for just over 2 years. Lately, my WiFi disconnects or slows down randomly, but there are no internet connection issues when I check the app. At this point, it is likely that I need a new modem. I've tried updating my modem via the site and it is not clear how I can do this. I go to xfinity.com/equipmentupdate and click update device. Then, it tells me that the Xfinity WiFi Gateway is added to my cart, but the "Next" button is greyed out and there's nothing I can do.