@D-Rider I tried adding the extender's MAC address to the white list in the main router. Then, I reset the extender to clear the configuration, and reconnected to the router using the tether app. Still, I receive a connection to SSIDName_EXT 'with no' internet. If someone from TP-Link could respond with advice, it would be appreciated. I'm about to return this thing for a refund. It's a just a brick without internet access.

  @D-Rider This is a FYI for all with a problem similar to the one I've been having, (or any other technical issue for that matter) If you want to get TP-Link involved directly, email them @ support.usa@tp-link.com (US) or support@tp-link.com (other). They contacted me, and even though I added my extender's MAC address to the main router, I still received no internet access from it. What I was instructed to do was to change the "SSID-Name"_EXT to something other than the main router, which I did. From there, I disabled all access control control on the main router, and was able to connect to the extender w/full internet access. After which, I re-enabled access control w/the extender's MAC address in the white list. Everything works as supposed to since. Internet access functions on both the router & the extender.