Hello @ConnieN
May I know do you try to log in RE in the Tether app, APP asks for a login password that is created by user when they first -time login RE.
Factory reset tp-link range extender and set it up again through Web page / Tether APP
RESET: with RE powered on, use a pin to push the reset hole until all the Leds flash once.
Later, please follow the instructions here to set up range extender :
How to configure Range Extender via Tether APP
How to configure my range extender via web GUI to extend an existing wireless network's coverage (new logo)?