I am having the exact same issue I cannot connect to my tplinkextender.net or tplinkrepeater.net or even the 192 string that should connect to it.

Place the range extender close to your router. Power it on.
Press the reset hole (about 8 seconds) using a pin to reset this device.
Wait for the device to reset and reboot. About 2 minutes. Don't press the RE button during this period.
Connect your computer and the range extender through a cable. Here your computer should be using DHCP. Also disconnect any wifi connections on your computer.
Open a browser and type in 192.168.0.254. If you can open the management page, configure it using Quick Setup Wizard.
After configuration, the RE button should blink for 0.5~2 minutes and then become solid on, which means the range extender is successfully connected to your router. The signal bars shows the signal strength between the router and the range extender.
Now wait for the computer to renew its IP address from the main router (1 minute at most). Then check the internet.
If everything is ok, move the range extender to another location. No need to configure it again. But one more thing you must take care is that the range extender cannot be too far away from the main router, the signal bars should be 3 bars at least.