A reply to this question was posted yesterday in another thread in which you asked the question: https://answers.unrealengine.com/questions/145328/license-details.html. I’ve copied the response below just in case:
“No royalties are due on work-for-hire fees. As long as the simulation that you create for this client is not made available to additional clients and is not sold, you should not owe any royalties under the UE4 EULA.”