Collabora in NextCloud only from LAN, not WAN accessible


as a ‘Windows person’, my Univention Server attempts are first steps in the Linux world, so I am presumably making some silly rookie mistakes and getting nowhere.

I have installed a Univention Server in Hyper-V under Server 2019 in a private network behind a FRITZ!Box in the current version (download on 26.02.2021). Probably my first mistake here was already to give the computer the name After I got a valid certificate via Let’s-encypt using the DynDNS name of the server, I set up NextCloud. From the local network NextCloud could be accessed without any problems, but when I then connected to NextCloud from an external workstation computer, I only got an error message. I found out that I have to add my DynDNS name to the NextCloud configuration file /var/www/html/config/config.php and since then I can access the NextCloud server from outside my local network.

But: while I can start Collabora from NextCloud within my local network without any problems, it seems that there is no communication between NextCloud and Collabora when I want to open a Word file with Collabora from my external workstation within NextCloud. Sometimes I get the error message that a connection is not possible. I assume that I also have to enter the DynDNS name I use somewhere in a configuration file for Collabora - but where …? Can someone help me to be able to start Collabora outside my home network within NextCloud?

Or should I better delete the current installation from the Univention server, set up the Univention server again and then assign the used DynDNS name to the virtual computer during the installation? Because I would expect that all conceivable other applications that I would like to install on the Univention server would also be linked to this problem and I would then have to continue modifying configuration files.