DavX5 to sync calendars and addressbooks. If you also want to sync tasks, also install jtx. Connect to server https://faircloud.eu (in case this does not work use the full URL https://faircloud.eu/nextcloud/remote.php/dav/ which you also find on the bottom left).If you want to sync an existing calendar and addressbook which is local on your smartphone, you need to first export and import them to the new calendar and addressbook.