That’s very good that service discovery is working.
The following works for me and friends of mine very reliable:
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delete eventuelly existing caldav accounts using your nextcloud-instance (example.xyz) from your clients (Mac/iOS).
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to add a caldav-account on your iOS-device:
go to “settings” --> “accounts+passwords” --> “add account” --> “other” --> “add caldav account”
As server enter only the domain name nothing else (in your example this would be “example.xyz”). Also enter username and password for the nextcloud user and hit “proceed” in the upper right corner. -
to add an caldav-account on your Mac:
go to “system preferences” --> “internet accounts” --> “other” --> “add caldav account” (I am not in front of my Mac right now - might be slightly differently).
Then choose automatic configuration. As username enter USERNAME@example.xyz (where USERNAME is your Nextcloud username) and the according password.