Trim and discard ends of plantains. Using a knife, slice a slit along length of plantain peel, avoiding flesh. Slice plantain crosswise into 1" rounds. Using your fingers or back of a spoon, remove peel from each slice; discard peel.
Lightly coat an air-fryer basket with cooking spray. Working in batches, arrange plantains in a single layer in basket. Cook at 400° until just tender enough to smash and remain in one piece, about 6 minutes.
Transfer plantains to a work surface. Using bottom of a cup, flatten to a disk.
Meanwhile, in a large bowl, stir water, 2 teaspoons salt, and ½ teaspoon garlic powder until salt is dissolved. Soak tostones in water mixture 5 minutes, then pat dry with paper towels. Spray both sides of dried tostones with cooking spray.
Working in batches, in air-fryer basket, arrange tostones in a single layer. Cook at 400° until golden and crisp, about 7 minutes; season with salt.
In a small bowl, whisk cilantro, yogurt, oil, lime juice, orange juice, cumin, and remaining ¼ tsp. salt and pinch of garlic powder until combined. Serve tostones with sauce alongside.
