On the Kalender page, select a calendar you wish to enable alarms for and click Bearbeiten.

Click on the tab Alarme and adjust the alarm settings as you like.

Click Speichern.

On the home screen of the device, tap Calendar.

Turn the device vertically and tap Calendars at the bottom of the screen.

Look for the calendar you wish to unsubscribe and tap on the icon beside it.

Make sure Event Alerts is ON. Finally, tap Done in the top right corner.