How do I get the xfinity app to ask for my new modem?

I initially installed the app on my phone and attempted to activate my modem.  It came up with an error (red exclamation) on my phone and said connection failed.  This stayed frozen on my phone and the only way I could get rid of it was to delete the app and reinstall.  Now it will not prompt me to enter the info for my modem to activate it.  How to I correct this so I can activate my new modem?