I just got off the phone with customer support. It looks like the microcell is tied to your IMEI number so if you change phones (same SIM), you'll have to reactivate (remove microcell and activate) your microcell in order for it to work properly. This only applies if it's tied to the main account. You can just readd a phone number if not. Hope this helps out.
I think this information is bad. It is not tied to the IMEI of your phone. I've switched my SIM to other phones and it worked with the microcell just fine. However when I got a new iPhone and activated the new SIM that came with it, it took a little time for that to start working. Reboot the microcell. Power airplane on then off. Also if using an iPhone do generel>reset>reset network settings. Eventually it'll work.