Need to activate replacement modem, but can't use Xfinity App

I received an upgraded modem in order to take advantage of increased speeds.  However, all procedures seem 

to require using an Xfinity App.  The current version of the app is not compatible either with my Android phone or my iPhone.  I tried merely replacing the modem, and that does not work so I put the original one back in.

I assume that I must activate the new modem, but how do I do that without the app?