You can manually subscribe to CalDAV calendars & CardDAV contacts via the mobile app by pressing the “Menu” (top left) and pressing “Settings”.
Then press “Add New Account” and press “Manual Setup”. Then scroll down to “Calendar or Contacts” and you will see a CalDAV & CardDAV option.
You then just follow the manual setup wizard with your Nextcloud or ownCloud URL’s.
If you don’t know the exact URL’s contact the technical support for those servers.
“Example thread link manual setup for adding a CalDAV Calendar to the Mobile app”