What IP address are you entering? Is it one from original setup or are you looking at what the router currently assigned it? I have found that during initial setup it may be on 192.168.1.250, but unless you set a static IP in the configuration, the router will dynamically assign it an IP address after reboot. This will lead to the extender having different IP addresses after power cycles. If you want to always know what the extender configuration page is, in the extender configuration wireless settings use static IP address, but make sure that any of your other wireless devices will not be trying to use the same one. Generally speaking "unused" IPs could be from 192.168.1.200 through 192.168.1.254.
