New XFi gateway

I recently upgraded my home system to a XFi gateway.  I used the XFi on my cell phone to activate the Gateway using the password on the bottom of the Modem.  In about 10 minutes the installation was complete and everything seemed in order, including the names I had used in the previous modem.  However, when I tried to sign on to the Gateway using my admin  ID and password I had written down, I could not get into the Gateway to make a change to the gateway.  Where did I go wrong?