Nextcloud Admin Panel Not available

Hello together,

I have installed nextcloud with docker and used this tutorial:

I have exactly followed all steps but when I want to login to the nextcloud admin panel or wizard that‘s not possible: site cannot be displayed.

My fritzbox has the correct port forwardings to 80 and 443 and my duckdns domain has been also configured and in the fritzbox I see successfully connected to dyndns.

Can anyone advise me why I’m not able to reach the admin or wizard page?