Loss of extended 5G when D7000 switched into AP mode

I have a TalkTalk router (wifi OFF) connected to my VDSL line, which is ethernet cable connected to my Nighthawk D7000. Further I have a Netgear EX3700 wifi extender providing wifi service in my more remote parts. I recently switched the D7000 to AP mode - everything went smoothly, except the 5G EXT from the EX3700. Connection to the 2.4G-EXT works perfectly, but connection to the 5G-EXT fails to obtain an ip address. I have re-set-up the EX3700 - it finds the 5G band it needs to extend, accepts the key, and says everything conneceted fine. However, trying to connect a wifi device to the extended 5G always fails on obtaining ip address.

Have I got a setting wroing, or has the EX3700 developed a fault in its ability to extend the 5G network?