Pretty sure the deal is a royalty if your revenue for the product crosses a certain threshold; I doubt a “free” product would cross that line.
If I make a model viewer as free using unreal, is there fee to pay? My company earns money by different libraries and projects.
You owe royalties for revenue made from products using the engine. If no revenue, no royalty.
You owe royalties for revenue made from products using the engine. If no revenue, no royalty.