Range Extender EX7000 - keep getting unable to join network

I haven't used this extender in a while - I went to set it back up, did the factory reset so the Netgear Genie does it's thing.  And every time I go through the steps - at the last step when it tell you to sign on the new network on a wifi enabled device - I get an 'Unable to Join the Network' message.  I've tried from the iphone and from my laptop.  Can't figure out the issue.  Anyone know of something else i can do?   I am not knowledgable about these things but had set it up previously and am generally capable of following directions... so I am at a loss.
It is the AC 1900 Wifi Range Extender Model Ex7000 
My modem/router that it is attaching to is the newest Xfinity one.  
Thank you for any assistance