Follow this, http://www.tp-link.com/en/article/?faqid=634

http://www.tp-link.com/en/article/?faqid=289 In ap client router mode, you can only connect to internet by cable plugged in LAN port of WA5210G.

I am confused... "after changing my SSID and security options and rebooting the device" Which one? The range extender? You know security options should be exactly the same as root router. You can...

You cannot reset it? In AP router mode, if you connect to its default Wi-Fi name successfully, you will be able to login.

Firmware is the latest version? Can you ping your router's IP, ping google.com or ping 8.8.8.8 if you cannot access internet?

You should be able to detect a TP-LINK Wi-Fi in default settings. Try to reset the unit, check status of Wireless light and try again. If the same, get another wireless device like phone or tablet...