The license for Unreal Engine does not allow publicly available images for the engine to be downloadable.
Thus, you need to use a restricted / private container repository, similar to how the Epic GitHub repository is private.
It may be that the particular repositories you’re talking about are private, and need credentials to pull, or it may be that they are no longer supported – I don’t know – but I would not expect a bare, anonymous “docker pull” to ever work for the Unreal Engine.