Planny supports two types of notifications. Local notifications for things like reminders and push notifications for teamwork.
To receive local notifications make sure to enable Push Notifications inside the iOS settings.
To receive push notifications make sure to enable Push Notifications inside the iOS settings.
Also activate background refresh in the iOS settings. Make sure that the app wasn't closed by swiping. After the app is completely closed you won't receive any notifications. Also, because of iOS power management, notifications might not always get delivered. In addition to that, notifications only work on iOS 12 or later. On older notifications they might not get delivered.