@Diffin 

By default LAN IP address is 192.168.1.1, unless it has been changed afterwards. Try using http://tplinkmodem.net when connected directly (wire or wireless) to the modem.y

If nothing helps - reset to factory defaults and try again:

"With the modem router powered on, press and hold down the WPS/RESET button for about 8 seconds until all LEDs turn off momentarily, then release the button."

  @Diffin 

If you use  TD-W8961N in bridge mode this is a known behaviour. Depending on your router adding a static route can resolve this - check this conversation.

If you want to use TD-W8961N as a modem router, you shouldn't configure bridge (modem only) mode. If you have an additional router, then TD-W8961N can be set in bridge mode and you could try setting a static route on the router so TD-W8961N WebGUI be accessible.

Keep in mind that TD-W8961N is a wireless ADSL modem router, it can't be used as a Wi-Fi router.