I bought an RE700X and it's working well except that, when configured "auto" (default), its own DHCP server starts running even though another, upstream DHCP server (on SONY NSD-G3000T) is working completely well without any other problem at all (there is absolutely no problem with signal strength and the like either), as shown by the attached screenshot.  I also took packet captures using Wireshark, all of which confirmed my understanding is correct.

This seems to be a known problem:

https://community.tp-link.com/en/home/stories/detail/1804?replyId=1502136

"In some cases, even if your TP-Link wireless device has connected to the front device with a DHCP server enabled, it may still think that there is no server in the network for whatever reason, and then it will turn on its own DHCP server. The clients will get IP addresses from it instead of the front device, which may cause the devices to be unable to access the front device’s network. In this case, you can manually turn off the Smart DHCP feature on your TP-Link device.

Log into the web management page of your TP-Link device, go to the DHCP Server page, disable the DHCP Server feature, and click Save to apply the settings."

Fortunately, I can manually configure the extender's DHCP server to avoid this problem, but the Japanese customer support insisted no such problem exists, even though I explained several times for a week or so showing more screenshots, and they finally refused answering my inquiries any more (I swear I have always been polite).  I hope TP-Link educates the "customer support" better.

P.S. For what it's worth, they even confused "DHCP" with "NAT" multiple times.