Preheat your oven or air fryer to 400°F.
Arrange frozen fries in a single layer on a baking sheet or air fryer basket. Bake until golden brown according to package instructions.
In a saucepan over medium heat, melt the butter and whisk in the flour until combined; cook for about 2 minutes.
Gradually add the milk while whisking continuously; stir in the salt, pepper, ranch seasoning, and cayenne.
Lower the heat to medium-low and cook until thickened (about 5 minutes), then stir in the shredded cheese until melted and creamy.
Pour the cheese sauce over the cooked fries and serve warm.