Our app is just showing events in the standard 'Calendar' app in your iPhone.

1. At first, launch the 'Calendar' app (not our app) from the home screen of your iPhone, and make sure that you can see all events in it.
2. Then, launch the 'Settings' app from the home screen, and go to "Privacy & Security" > "Calendars" > "Calendar Op.2".
3. Select "Full Access".
4. Launch our app, and go to Settings Page > "Select calendars to display" and turn on all calendars you'd like to see in our app. That's all.

And also, If you had the misfortune to lose your calendar data during an iOS update, or if you accidentally deleted it, Apple offers a way to restore it. Please refer to the Apple's official support page below in this case.