2 airport extremes, one extends the main network but doesn't provide internet over wired ethernet connection
I do this all the time. 


Connect any of the normal Ethernet from the main station to the source Ethernet on the second station. 
Set the second to bridge mode
Have each broadcast the same network, don’t extend, just set up the same or different SSID / network names to suit your preferences. 


Having both create a wireless network works best for me. Until such point you get newer hardware like UniFi Dream Machine (or pro) or perhaps another ubiquiti line or like product.