@0-_ 

When you access a device through the Tether APP for the first time, you will be asked for a password to login, that password is the same as the Router's web management interface password, not your TP-Link ID password or your Wi-Fi password. If you have forgotten the admin password of the Router, you will have to reset the Router back to factory default.

You could also check if you could login the web interface of the router.