The default FollowerEntity does not have an authoring component, so I don’t think it will work in a subscene. If you put it in your base scene the Entity should be generated and connected to the gameobject. You should be able to then write ECS Systems to interact with it’s components.
If your looking to do completely ECS with your FollowerEntity in a subscene then you will need an authoring component like this one I made. Working with it is a bit different because it doesn’t have all the helper functions that the FollowerEntity script provides, but most should be fairly easy to figure out what component you need to access to achieve the same result.