We have a handy support page to help with Netgear WiFi extenders, is is located here https://help.familyzone.com/en/support/solutions/articles/5000801936-pairing-netgear-extenders-to-home-zone. If you are still experiencing issues after this, I do suggest submitting a detailed ticket to out support team, and they'll be more than ready to help out.
Customer Service Representative
I easily connected my old Netgear ex6100 extender, which although uses WPS, can also be connected to the FZ box WiFi using the netgear genie function. I did a factory reset (use a pin) on the extender to make it forget the old unfiltered NBN router WiFi, and then followed the (genie) steps of the installation guide to connect it to FZ (and thus extend FZ WiFi in distant rooms): https://www.downloads.netgear.com/files/GDC/EX6100/EX6100_IG_27Nov2013.pdf