Add the light mayonnaise, dill pickle relish, dried dill, lemon juice, salt, and pepper to a small bowl. Mix together, cover, and refrigerate while you get the fish ready.
Place the fish fillets in the bottom of an air fryer. Cook at 400 degrees F. for 12-14 minutes, flipping halfway through. (If you don't have an air fryer you can follow the baking instructions on the box.)
Open the bun and add 1/2 slice of fat-free American cheese to the bottom of the bun. Top the cheese with a fish fillet.
Spread 1 tablespoon of tartar sauce on the inside of the top bun and place it on the bottom bun.
Serve the sandwich immediately after assembling to ensure the fish remains crispy and the cheese perfectly melted.