[lhpz]

The OS would notify the app that its IP address has changed. The app can then send this new IP to its notification server, for future notification to arrive at the correct IP. No need for an Apple notification server here.

This would still require some cooperation from Apple, for the OS to receive the notifications and dispatch them when the app is not running. A long time ago, when there was no outrageously dominant players, the Internet industry would have produced an open communication protocol for this use case. Using the same protocol and the same notification server for both Android and iOS would be nice. Sadly, this is unlikely in today's Internet.