Setup Airport Express extender via Ethernet

Have you tried doing a factory default reset on the Airport Express first to set it up as a new device?

Factory Default reset (using reset button)
If the device isn't accessible from AirPort Utility, the Factory Default reset can be accomplished by using the reset button.

1. Disconnect the base station's power cord.
2. Press and hold the reset button with a pen, pencil, or straightened paperclip. While continuing to hold the button, reconnect the power cord. Keep holding the reset button until you see the status light (LED) start to flash amber rapidly, which should occur after about six seconds.
   
   

Then you want to configure a roaming network.
https://support.apple.com/en-us/HT204616

 

in order to set an airport extreme as an extender " extend a network " over ethernet you must first reset the airport
to factory defaults and use airport utility 5.xx or 5.xx for windows. this function was removed from airport utility 6.00 as far as i remember

once it is set to share via ethernet or wifi, you cannot change it to something else without first resetting it to factory defaults.

psyopwarload's post is very good. i think thats the correct one, but i am pretty sure in order to do it, you need the older version of airport utility.

another cool thing with setting it up that way over ethernet is each basestation can be on a different channel.