How do you save game data to iCloud using CloudKit? (or Firebase?)

This is the single topic I can’t wrap my head around. There are settings in Unreal to enable CloudKit, but once I do my game crashes on startup when built to a phone. I can’t find any documentation on the CloudKit integration and I can only find one thread from 2020 about this topic - following their named steps still makes my App crash ( → link to thread).

I want to save some images and variables to the iCloud. Did anyone successfully implement this? Or do you have recommendations on how to do it otherwise? Unfortunately setting up an account and server system with e.g. Firebase is new to me and also there I can’t find any starting points online.