@Miskatzalil 

Hi, thanks for posting question here.

When you access a device through 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, not your Wi-Fi password. If you have forgot the admin password of the Router, you will have to reset the Router back to factory default.

If you are sure that the admin password is correct but the issue remains, please try the following suggestions:

    1) Update the Router's firmware through its web management interface, go to Advanced > Advanced > System Tools > Firmware Upgrade to check for updates.

    2) Update your Tether APP to the latest version.

    3) The Router's LAN IP address should be in the same Subnet as your phone IP address.

You may also contact TP-Link support for direct help.