The iOS calendar app doesn't sync iCloud calendars anymore.
Example: I add calendar events on a mac, which uses the same iCloud calendars, and add a notification for in 5 minutes. In the past, my iPhone would notify me about this event though the set notification. This doesn't happen anymore as I now have to manually open the calendar app on my iPhone in order to make it sync the calendar events.
How can I fix this?
So the actual solution was to go to:
Settings > Calendars > Accounts > Fetch New Data > [My Account name] >and here tap on "Push".
Selecting "Push" in
Settings > Calendars > Accounts > Fetch New Data
was not enough.
You will need to set your calendar account(s) to “push” updates to your iOS device, or “fetch” updates on a regular interval.
The following steps apply to iOS 11.2.1 for iPhone. If these instructions don’t work for you and you’re running the same or a later version, please edit or comment as appropriate.
1. Open your Settings app.
2. Scroll to “Accounts & Passwords.” Don’t select “Calendars.” (Not intuitive, I know.)