In the Wordpress Manual i found the Information:
Currently, FreedomBox does not provide an easy way to change the domain name once the app is installed.
What is the hard way to do it?
Problem Description
after changing the ISP and Domain i cannot reach the Wordpress Admin Page
During the first setup of WordPress (where we create a WordPress admin account), whatever is the domain name with which we access WordPress (the URL in the browser), it will taken and configured in WordPress database.
We don’t know how to change it after that. It must be documented somewhere. Perhaps there is a setting for it in the admin panel. Perhaps there is a command line tool or a database query to change this. It likely won’t be too complex. If someone looks up the documentation for it, we can implement it as a feature in FreedomBox. It was simply out of scope for the first cut implementation of the app.