[sneak]

There is no way for your selfhosted xmpp server to deliver notifications to an iPhone unless you are the publisher of the xmpp client app you use (which requires doxing yourself to Apple and paying $99/year if you don’t want to sign new binaries every week).

The server support is irrelevant; this is an iOS/APNS issue.

[MattJ100]

The notifications are relayed via a server run by the app developer, who pays the fee and has the key.