@Blister 

Hello，

If the extender has no internet access after the configuration, please check the RE light or signal Led on the extender. Then please refer to this article to do the troubleshooting:https://www.tp-link.com/en/support/faq/2234/

Note: When checking the router's wifi password, please also confirm that the authentication type and encryption type is Auto on the main router.

If it still not working, please tell me what you have done, thank you!

Have a nice day!