It took me days to get this to work, so hope it helps others.
I have an AX50 which I wanted to extend with my new RE700X. Initial set up went fine and when relocated all the lights where blue (i.e. both 2.4 and 5 Ghz working). I could connect my laptop to the RE700X but no internet.
I tried to change settings on the AX50 including disconnecting the IP & MAC binding. That did not work. I reset and rebooted the RE700x countless times. Also tried both the methods described in the support document https://www.tp-link.com/uk/support/faq/2965/ and mentioned elsewhere on this excellent forum. Always getting the same result - no Internet and usually showing problems of not finding the DNS. 
Eventually I thought I needed to go back to basics. I disconnected Access Control on the AX50, reset the RE700X (i.e. Network settings > Auto) and it worked - connects fine to the Internet 
So now I have the MAC address of the RE700X added to the Access Control on the AX50 and enabled. I have Access Control also set on the RE700X and so I am back to normal with the added benefit of the extender working fine.