EX3700/AC750 WIFI Extender setup failure

I am getting exactly the same behaviour as the OP.  I am definitely setting it up as an extender. Each time I try to set it up, it gives me a list of Wifi networks, I select my 2.4GHz network and uncheck the 5GHz option. After a short while it prompts for the password, which I enter, then it asks for names and security settings for extended Wifi networks.

Aside: It seems odd to me, but at this point in the process, it doesn't seem to be connected to my network - at least, there's nothing new in the list of DHCP leases.

Anyhow, I accept the default settings for the extended networks. It says "We are applying your settings", progress whirrs for a bit, then the network connection drops, and shortly after, the two extended networks appear. When I join either, it says "You have created an access point but we did not detect an Internet connection" and invites me to start again. It still hasn't connected to my wifi.

I don't have any MAC filters or ACL lists on the router. My network does have an internet connection (I'm using it to type this). I've been around the loop four times. I tried a factory reset which had no effect.

I also tried extending my 5Ghz rather than 2.4GHz network. This time I only got as far as entering my network password, before it whirred for a considerable time, occasionally warning me that it was about to send information insecurely, before eventually announcing that "The Extender was not able to connect to your existing network". It's worth noting that when I selected my 5GHz network, the device told me the signal strength was 99% (it is very close to the WAP!).

Any suggestions? In particular, does anyone know how to get any kind of information about what's going on from this device?