What is the actual model number of your extender please e.g. EX7000?
The URL www.mywfiext.net can only be intercepted by the extender, and hence provide access to the Admin pages, if the wireless device you are using is associated with the extender. A common issue is that the wireless device is still associated with the base-station. Basically you need to check the current WiFi settings of the computer and check that it is connected wirelessly to the extender.
