The blog shows a VM. You installed a container. Fail2ban can have issues with this.
Additionally, i guess the installation failed since you used Debian 12 instead of Debian 11.
i you can the ip with commad
ip r
But i bet if you open that IP in a Browser, nothing will show.
It’s really important to read everything carefully.
Thank you!! You are absolutely right. I reinstalled Debian 11 to the CT and read again carefully with success! Only change in the GitHub is the requirement in Debian Minimal installations to install sudo with : apt install sudo before the final command.