@johnwhelan 

Hi,

If you don't want to go throught the trouble of logging into the GUI of the AXE75 to switch it into "Satellite" mode, then you can alternatively reset the AXE75 to factory defaults by pressing and holding (about 8 seconds) the small "Reset" button on the rear side of the AXE75. You will know whether it worked when you see a distinct repsonse from the router's LEDs.

This reset will also put the AXE75 into the mode that allows it to be added as a "Satellite". (But do not try to log into this router now, as this would break the whole procedure!) 

Now the AXE75 should be automatically added as EasyMesh Satellite after you plug the Ethernet cable into one of its yellow LAN ports. (In best case it will happen almost instantly, but it could also take up to one minute. I've had varying experiences.)

Edit: I just noticed that you wrote another post about about this yesterday. As I indicated in my reply to your post, do not just assume that your newly purchased Archer AXE75 already comes with an up-to-date firmware out of the box. In case the firmware is older than 1.2.0, then you need to update the AXE75 first before EasyMesh Ethernet Backhaul will work. Whether you do this manually by downloading the firmware from TP-Link's website or via the "Online" update (which would require you to connect that AXE75 to the Internet) is your own choice.