DIAGNOSIS Your device is not on the router's local network. You're trying to reach it via a disconnected WiFi or wrong IP.
FIX Connect your computer directly to one of the router's LAN ports with an Ethernet cable. Open a browser and type 192.168.0.1 into the address bar. That should work.
IF THAT DIDN'T WORK The router's IP might have changed. On a connected device, open command prompt. Type "ipconfig" (Windows) or "ifconfig" (Mac). Look for "Default Gateway". That IP is your router's address. Type that into your browser.
HEADS UP tplinkwifi.net only works when connected to the router's WiFi. It does not work over the internet or a VPN.