Fill a large bowl with ice water. In a medium bowl, whisk 2 cups of the flour with 1 teaspoon of salt. Whisk in the club soda until the batter is smooth. Set the bowl in the ice bath; let stand until chilled, 10 minutes.
Meanwhile, in a small bowl, combine the cumin with the oregano and paprika. Season the fish with salt and pepper and sprinkle with the spice mixture. In a large bowl, gently toss the tomatoes with the onion, jalapeño and cilantro and season with salt.
In a large, deep skillet, heat 2 inches of vegetable oil to 350°. Place a rack over a rimmed baking sheet. Working with two at a time, dredge the fish fillets in the flour, shaking off any excess. Dip the fillets in the batter and fry, turning once, until golden and cooked through, 4 minutes; transfer to the rack. Repeat with the remaining fish.
Arrange the rolls on a work surface cut sides up. Top the bottom halves with the lettuce, fried fish and tomato mixture. Close the sandwiches and serve right away.
These sandwiches are great with a crisp Chilean Sauvignon Blanc.