Well, I do not use any wifi extender at home, but two friends of mine are using that device (not Netgear).
One of them, using a TP-Link device was able to use the WPS functions (both on the main wifi router and on the extender) and the TP-Link started working immediately. The other, using a different product (I don't remember the product's brand) had to contact the manufacturer's support team and they were able to solve his problem, which was related to the dual band (2.4 or 5 GHz) configuration on the main modem-router.
If your iPhones were able to connect to your main wifi network (before you installed the extender), I don't think the problem is coming from your iPhones.
Regards
Giulio