WN2500RP connecting issue

Are you certain that your extender is successfully connected to the router?  Look at the 2.4 GHz and/or 5 GHz lights on the extender.  Green indicates a good connection, yellow indicates mediocre, and red indicates poor.  If they are both unlit, then the extender is not connected to the router.  You can also check the connection status by logging into the extender and going to the Status page under Maintenance.  Make sure the Connection Status to Existing Network is Connected.  The extender should have obtained its own IP address from the router.  See page 32 in the manual for an example.

When you connect a device to the extender, does the device receive an IP address?

Do you have Access Control enabled on your main router?  If yes, then you have to do some extra work to permit devices connected through the extender to reach the router.  See the answer to question 6 in the manual.