EX-2700 - Iphone se and Iphone 5s

After a power outage our Iphone Se and 5s would repeatedly state password not recongized when trying to reconnect to our Netgear ex 2700 wifi-extender

We are using the same password on the extender as on our Vodafone internet hub

Umpteen resets of both our router and our extender have failed to cure the problem

Also tried changing just the extender's password but that also failed to be recognized by the iphones

Android devices and Windows Laptops all connect fine to the extender - but the Iphones will not

Extender's firmware is up to date 

Extender setup reset via Netgear Genie umpteen times as well

Totally exasperated as to what to try next.......