Hi @MarmiteLegend,

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 Range Extender's web management interface password, not your TP-Link ID password, not your Wi-Fi password.
The default login username and password are both admin. If you have changed the admin password and forgotten the admin password of the Extender, you will have to reset the Extender back to factory default and start from scratch.
If you are sure that the admin password is correct but the issue remains, please try the following suggestions:
    1) Update the Range Extender'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 Range Extender's IP address should be in the same subnet as your phone IP address.