I can access the AP's login page, but why is the default password incorrect?

Zyxel_Nami

If you can access the login page but the password is incorrect, it's possible that the AP was previously registered on Nebula.

In this case, you should use the password available in the Site Settings on Nebula. Please refer to this FAQ for more details:

https://community.zyxel.com/en/discussion/18764/what-password-should-i-use-to-access-the-web-gui-of-my-ap-managed-by-nebula