iOS push notifications not delivered after allowing permission at login

Customer reports that push notifications on iOS are not working. When logging in, they receive the prompt to allow notifications, but no notifications arrive afterward.

Not yet known (requested from the customer): iOS version, whether they use the Gathrings app or the website in Safari, whether they tapped Allow, and whether notifications are enabled for the app in iPhone Settings > Notifications.

Source: messenger conversation with Alexis Gingras.