EX2700 WiFGi Range Extender - setting up afresh with new ISP's router

We have recently changed ISP and installed their new router - with a fibre connection at last.  I am trying to set up the WiFi extender again, based on the original instruction sheet, and have suceeded in creating the extender's own network ID with the -EXT extension.  However, on going to the mywifiext[dot]net link, I am presented with just the login screen, withough the NEW EXTENDER SETUP button to which the instructions refer.  And when I try to log in with the email address applicable to my registered Netgear account, which is unchanged from the previous setup, the etender rejects this as "Invalid username".  I tried the Username and Password Help link, supplying the correct challenge answers, and was presented with precisely the details (username and password) with which I was attempting to log in and getting rejected.  How do I solve this (it woudl help if there was a new extender setup option after all)

Any suggestions much appreciated, thanks