For the purpose of documentation, and because it tooked me 2 hours to solved. The main idea was to just import a external CalDAV link (ics) in to our Nextcloud instance.
Actually it wasn’t working well, just because the default refresh rate of nextcloud is really slow… So to speed it up you can just do this on your Yunohost instance:
sudo -u nextcloud php occ config:app:set dav calendarSubscriptionRefreshRate --value "PT5M"
The format of time interval (here “PT5M” meaning every 5 minutes) is following this rules: An Introduction to the PHP DateInterval Class (webfx.com)