If the user declines (e.g. byu accident) permission for camera access during startup of an ARKit application, there is not way to request permission again. In Android there is, but I cant find anything related for iOS. Is that the case?
iOS has a method for this, but this is again one of many things that are not available in Unreal Engine.
For those who master the art of combining Swift or Objective-C with UE4-C++, this is the missing function:
The rest of us has to ask the user to inconveniently go to iOS Settings > Privacy, enable permission and come back to the app.