I am in a warehouse with 100ft of cat 5e Ethernet cable. I just hooked up to my Comcast business class gateway. My 100ft of cable goes from their gateway device to my computer just fine. 20mbps. However when I try to hookup to my router I get no wan light (tried 3 different routers). So the Ethernet cable works great directly to my computer but the router shows no wan light.
So...I bring a laptop - hookup the router to the gateway with a 5 foot Ethernet cable...plug my laptop into the router and the wan light shows up with 20mbps.
Usually I would say the 100ft Ethernet cable needs to be replaced but it works directly to the computer. In addition, from what I read only when an Ethernet cable reaches over 300 feet is when problems start to occur.