FWIW. I currently employ an AT&T Microcell in my network configuration. It needs to be connected via Ethernet to a router or to an Ethernet switch that is connected to a router. It does not support a wireless connection for Internet access.
An 802.11n AirPort Express can be configured as a wireless Ethernet bridge. This would allow for it to connect to a wireless network and have its Ethernet port active for the Microcell. However, the key would be that it would need a wireless network to connect to. It sounds like you are getting Internet service via a Microwave connection.
I finally solved the problem with the help of an excellent ATT tech. Here is some background. My internet provider uses line of sight tower to dish technology. An ethernet cable runs into the house and connects to a powered device called an Injector of Power over Ethernet. From this device there is an ethernet cable connecting to my Airport Express. The question as to whether my Airport Express was bridged or not bridged was a non issue because (as an Apple tech told me) the Airport Express is not bridged (but can be). The first ATT tech I spoke with said that the Micro Cell would not work without a modem and, as a result, she offered no further help; however, the next ATT tech I spoke with, went the extra mile and wouldn't give up even though he originally felt that the lack of a modem was a deal breaker. The problem he discovered was that the Micro Cell needed to be re-registered. I hooked everything up and ten minutes later I had 5 bars. Thanks for your thoughts on the subject.