So I can set up my new extender using WiFi which I have done, but the connection is still fairly poor due to the thick wall dividing our home. 

The extender is situated right by an Ethernet port in our kitchen that is wired directly to the main router in the other half of the house. I have tried every which way to set up the extender using the cable and not the WiFi but it will only work off of the WiFi. The last thing I tried was setting up the extender with the tether device app via WiFi, and then once set up, plugging in the Ethernet. However, as soon as I did so, the signal light started flashing red and the extender lost signal. The Ethernet port 100 works correctly as if you plug a laptop into it it works lightening fast with no dropouts. Can anyone advise what I need to do to set up the WiFi extender via Ethernet as opposed to WiFi before I take the damn thing back? Telephone support team said it should just connect automatically, but clearly this isn’t the case. Thanks in advance :)

P.S I HAVE NO IDEA WHAT HARDWARE OR FIRMWARE ITS RUNNING ON