After wasting my time for 30 minutes trying to get www.mywifiext.net to work, here's what I did:
--I had already set up the WN3500RP using WPS.
--Connected to the WN3500RP via Ethernet (i.e. ran a CAT6 cable from my laptop to the Ethernet port on the WN3500RP).
--Went into my router's DHCP table to get the IP address of the WN3500RP.
--Entered the IP address as the URL (e.g. 192.168.1.144).
--Changed the WN3500RP to use a static IP address outside the range of my router's DHCP list (e.g. 192.168.1.163). That way it will always be at .163 so I don't have to hunt for it every time I want to access the admin page.
--Entered the new static IP address as the URL (e.g. 192.168.1.163).
--Updated the firmware and customized the WN3500RP.