Microcell iPhone 3GS

Like many, I have poor AT&T reception in my home. I bought a Microcell to boost my signal. A really basic question as I am attempting to setup. I have an Airport Express router which only has one ethernet port which is being used to connect the router to the Time Warner Cable cable modem which also has only one ethernet port being used to connect it to my router. Since the Microcell connects to either your router or modem via an enternet port, I assume I need either a new router or modem with more than one ethernet port in order to use the Microcell, right? Any workarounds I may be overlooking or thoughts? Thanks!
MacBook 13" aluminum (2.4GHz, 4GB RAM) Mac OS X (10.5.8) iPhone 3GS