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Hi @K+S Spälti
Thank you for posting your question in the Microsoft Q&A forum.
Is the calendar in Team site or Communication site?
Just for clarification, you created a SharePoint Calendar within List or you are using SharePoint Events in Site Contents?
Calendar in Microsoft List
SharePoint Events in Site contents
You can try using SharePoint app for mobile device (for both Android and iOS).
About SharePoint mobile app - Microsoft Support
The app supports accessing SharePoint sites, including calendars. Features include viewing calendar events in month, week, or day views, adding, editing, or deleting events.
Note: This is my recommendation based on what I researched and conducted, if you prefer using a third-party app, Microsoft is not responsible for the accuracy, security on the app, please use at your own risk.
I found 2 apps that can access SharePoint contents, SPConnect and SharePlus - SharePoint Mobile. Both apps are available for Android and iOS.
However, if you create a calendar with List, these 2 won't work since Calendar list view doesn't support mobile device. It only supports web view which SharePoint app is using.
I hope this information helps.
If you need any other information, please feel free to reach out.
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