Integrate external Calendars in Nextcloud

I have a UCS/Kopano server here and would like to use Nextcoud. But this requires that the Nextcloud calendar and also the contacts get their data from Kopano. I have found instructions on how to export the Nextcloud calendar via Z-Push/EAS, but I need it the other way around. Nextcloud would need to pull the data from Kopano (however). I like the apps in Nextcloud. But without a way to integrate with Kopano, they are absolutely useless to me. Is there a solution here? I’ve been looking for it for quite a long time and can’t really find anything useful. Maybe I am just incapable of searching…

Am sinnvollsten ist meistens sich für eine Seite zu entscheiden und die Kalender dort zu führen… Ansonsten hilft vielleicht das hier weiter: