The object you are looking for is called “Lumen Exposure Manager”.
You can search for this Blueprint in the content browser, more specifically in the “Fortnite” folder.
The “Lumen Exposure Manager” has two post process components attached, that allow you to drive different settings depending on if the application runs on a Lumen or Non-Lumen device.
They are called “LumenPostProcess” and “NonLumenPostProcess”. It can work on top of the “Fortnite Time of Day Manager” and “Day Sequence Device”. The “Environment Light Rig” does have this functionality built in, too. So I would not recommend mixing the “Environment Light Rig” and the “Lumen Exposure Manager” to avoid confusion.
If your intention is to use the “Fortnite Time of Day Manager” and not build your own lighting, this logic is automatically integrated.