DavX initial Sync

I set up a DAV-X Sync with my newly installed Nextcloud.

The sync copies a newly created contact to my Android phone, and it syncs any change I am doing on this contact back to Nextcloud.

However, all 300 Contacts I did have on the phone before I set up the NC in DAV-X are not synced.

Any advise?

(Sorry, I could not find the version of my NC instance anywhere, but it was installed 2 weeks ago from the Ubuntu LTS repository).