Reconnect EX6150 Extender

I recently set up a new router with WEP security.  I have an extender that was working well with the previous router.  How do I get my existing extender to link up with the new router?

If I turn the extender on, connect to it with my computer and type the www.mywifiext.net, I am sent to the NETGEAR sign-in page and my email and password are not accepted (probably because the extender is no longer connected through the router to the internet and therefore can't sign me in to netgear).

I've tried the factory reset button, no joy.

Any suggestions?