@Lengeldu
Sorry for the late response, you could factory reset extender and install it from scratch:
If Tether does not found range extender, connect your mobile phone to Extender's WiFi (turn off VPN on the phone if there is any)
Also make sure your range extender supported by Tether app.
https://www.tp-link.com/tether_compatibility_list/
RESET: With the extender powered on, use a pin to press the RESET button until most Leds flash once.
Later please follow the instructions here to set up range extender :
(How to configure Range Extender via Tether APP)https://www.tp-link.com/support/faq/357/
(How to configure my range extender via web GUI to extend an existing wireless network's coverage)https://www.tp-link.com/support/faq/1400/