In a large bowl, add the flour, cornmeal, baking powder, salt and cayenne and whisk to combine. Whisk in the beer, being sure to remove any lumps.
Cut the catfish fillets in half lengthwise to make 2 strips. If desired, cut into bite-sized pieces. In a small bowl add the salt, pepper and granulated garlic. Season the fillets with some of the salt mixture. Reserve the remaining salt mixture for another use.
Dip each strip in the beer batter and add to the fryer, a few at a time, moving them around so they don't stick. Fry until brown and crispy, about 3 to 4 minutes. Remove from the fryer to a serving platter. Repeat with the remaining fillets and season the fish with lemon juice.