Solution for Microcell with Xfinity Modem & Linksys router
I had trouble activating my microcell--it was stuck doing NOTHING, after following directions carefully three or four times. AT&T secondary support verified I was doing all the required steps, and sent me to my ISP. It took a while to get to their (really) technical support, but after I did, Joe listened to the requirements AT&T posted on this page: AT&T Microcell Firewall and Router Settings for Advanced Installations. He took a different approach, and changed our Xfinity Modem, connected directly to our Linksys Wifi & Ethernet router, to "Bridge Mode."
After doing all the power-down, restarts of the modem, router, and Microcell, I then disconnected the microcell from my AT&T account, and re-linked it, starting the process again. SUCCESS!!!
I'm posting this to help others, rather than to ask a question.