Did you install iOS Beta versions on your devices?
If yes, in order to get the notifications working again, you will to downgrade your firmware to the latest official version.
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The iOS, iPadOS and macOS betas come with the built-in Feedback Assistant app, which can be opened from the Home screen on your iPhone or iPad or from the Dock on your Mac. The Feedback Assistant app is also available from the help menu of any app by selecting Send Feedback. If you’re running the tvOS, HomePod software or watchOS beta, you can submit feedback through the Feedback Assistant app on an enrolled iPhone, iPad or Mac. When you experience an issue or something does not work as expected, send your feedback directly to Apple with Feedback Assistant.
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copied from: Apple Beta — FAQ
Apple Beta — Restore Shipping Software