Your phone and iPad support dual bands? How did you confirm it? Any other 5GHz Wi-Fi networks around that you can detect and connect to? What kinds of phone? What's the version of your phone and iPad?

Did you contact TP-LINK support team? Your issue has to be further looked into according to my understanding. http://ticket.tp-link.com/index.php?/Tickets/Submit

Someone solved it with beta firmware. http://forum.tp-link.com/showthread.php?80013-Tp-link-3040-for-bsnl-evdo-ac8700-zte

Try to disable Wi-Fi security and change the SSID. If MAC connects, try to secure the network again to test. If it doesn't, you may reset the router to factory defaults and reconfigure it.

Default port number is 80. Remotely, you have to login with WAN IP address obtained on router page.

Did you check IP address, default gateway obtained on android devices? Upgrade the range extender as well, reset and reconfigure it. If the problem continues, you are suggested to replace it.