Hi CreativeCakeLady and welcome to the forum
I am not familiar with Apple products, but I gather from Amazon that there is
Base station that has two Ethernet ports, and
Airport Express that isn't really a router by itself. So what do you have?
In most cases router has a wired port (that allows it to actually
route). I know there was Fon that was an exception, and Apple always is an exception... but it's still hard to believe. Do you have extra Ethernet ports on the modem (which would indicate that modem has a built-in router)? If modem has a built-in router, then you could plug a simple switch where the long cable comes up from under the carpet, and plug Airport express in one port, and OBi into another. Or, as giqcass suggested have an OBi202 with WiFi