Acessing existing router and other devices on network

I appreciate your response but that answer doesn't solve my problem.  My laptop and T-mobile gateway are two floors away from the router and supplimentary switches for all of my ethernet connections.  I only have one Cat5e cable running through ther walls from the network closet to the office where the laptop and gateway are located.  Previously, when I was using cable for internet, my laptop was connected via ethernet, to the router through that in-wall cat5e cable.  Now the T-moble gateway uses that single cat5e cable to connect to the router two floors below.  Because the t-mobile gateway is now using the cat5e cable connection to the router, I have lost the ethernet connection to my laptop.  My current solution is to connect my laptop directly to the t-Mobile gateway via cat5e cable an using the 2nd ethernet (RJ45) port at the back of the gateway.  

My issue is that the T-moble Gateway is not acting like a switch and is not allowing me to access my router and home networked devices.  Is there a way to configure the T-Mobile gateway so that it acts like a network switch and allows for one connected device to have visibility and access to another connected device, when both devices are connected to the gateway by ethernet (hardwired devices)?   I prefer not to use wifi connections for my computers when connected to my home network.