DIAGNOSIS Your computer isn't on the same network as the router, or you have a previous network profile interfering. The tplinkwifi.net address only works when you're directly connected to the router's network.
FIX First, connect via Ethernet cable to one of the router's yellow LAN ports for setup—it's more reliable. If you must use Wi-Fi, look for the default network name on the router's label (like "TP-Link_XXXX"). Connect to that. Then try the IP address 192.168.0.1 instead of the web address. Use "admin" for both username and password.
IF THAT DIDN'T WORK Disable any VPN or proxy software on your computer. Also, try a different web browser (Chrome, Edge, Firefox) in a private/incognito window to rule out cached data or extensions blocking it.
HEADS UP If you've used a TP-Link router before, your computer might try to connect to the old network name with the new password and fail. Forget any old "TP-Link" networks in your Wi-Fi settings before trying again.