This gave me some serious headaches. To sync, make sure the calDAV app is installed on Hubzilla, then go to Accounts and Passwords on the iPhone settings and add a new CalDAV account with these features:
I set the URL in the Advanced Settings page.
Then it does a verification check and should show a bunch of checkmarks. To get calendars to appear, at this point I had to de-select the Calendar sync option in the account settings, then click OK to delete the calendars from the iPhone, then reactivate the sync option. Magically it loads the calendars. I expect that you have to repeat the de-activate re-activate sync step any time the account settings are altered.