Troubleshooting iOS contacts and calendar sync

Hi :

I am trying to get my kids’ iOS devices to sync their calendars and contacts to my locally hosted Nextcloud server. The server is version 21.0.4 and the iOS devices are definitely above version 12 so I setup NC with Let’s Encrypt and even made my server’s IP static (I think I read that somewhere). Still no joy. iOS fails with a generic error and even offers to connect with HTTP which doesn’t work either.

How can I troubleshoot the iOS device and get a better idea of what exactly is failing?

Oh, and my wife and my Android devices work fine with DavX5.