Can't extend Network with ethernet

I have a Motorola Surfboard modem and a Time Capsule Base station with Airport Extreme as my network extender. There seems to be a new airport utility, because when I set up the Airport Extreme it automatically sets it up wirelessly to the same network. I do want it to be the same network, just with ethernet. The Apple Rep told me to just plug in the ethernet, and it would detect it automatically. It does--but the wrong way--the network goes down when I plug in the ethernet cable. Don't understand. This worked well for three years. (Our Time Capsule Up and died yesterday and just got a new one. Airport Extreme two years old).