Hello   @motero99 

• To let Tether app detects your device, confirm the mobile device installed Tether APP and the Range extender are in the same local network, 

turn off Mobile data and VPN on the phone, connect phone to Range Extender or router's Wifi network and open Tether app again

• The login password was created when you first time access  'tplinkrepeater.net' the web UI of range extender or first time login device in Tether APP. 

If the login password says incorrect, try to factory reset the range extender and set up a new login password for your range extender. 

Factory Reset Your Extender:

• RESET: With the extender powered on, use a pin to press the RESET button (pinhole) until all the LEDs flash once.

Here is a  Configuration Guide including reset step that may help : TP-Link Range Extender Setup (Range Extender Mode)