Should I use "primary CalDAV address" or "iOS/macOS CalDAV address"?

Trying to sync up iPhone with Nextcloud calendar.

Which server address should I input into iPhone: “primary CalDAV address” or “iOS/macOS CalDAV address”?

I tried inputting primary address and the iPhone and Nextcloud calendars sync if a new entry is added to calendar. But all the pre-existing iPhone calendar entries are not syncing with the Nextcloud calendar. For example, I have a calendar event in my iPhone calendar for March 1. But this does not show in the Nextcloud calendar.