@Bexks
Hi, thank you for posting on TP-Link Community.
May I know which password you mean?
1. If you mean the login password of your TP-Link device on the web interface, please try "admin" in lower case. However, if admin doesn't work and no other possible password, please reset your TP-Link device to default settings by pressing and holding the reset button for 10s.
2. If you mean the Wi-Fi password of your TP-Link extender, after setup, it will have the same wireless password as the main router, so please use the Wi-Fi password of your main router to have a try.