Whisk garlic powder, salt, brown sugar, paprika, chili powder, pepper, cumin, onion powder, cinnamon, and coriander in a small bowl to blend. DO AHEAD: Spice blend can be made 1 month ahead. Store airtight at room temperature.
Preheat oven to 300°. Rub ribs all over with spice blend and wrap each rack individually in foil, crimping seams to seal tightly. Place both racks on a rimmed baking sheet. Bake ribs until fork-tender, about 3 hours. DO AHEAD: Ribs can be made 2 days ahead. Open foil packets and let ribs cool completely. Rewrap tightly and refrigerate on same baking sheet.
To continue: Preheat oven to 425°. Cut racks between bones into individual ribs. Divide ribs between 2 foil-lined rimmed baking sheets and bake until heated through and golden brown, 15-20 minutes.
Just before serving, warm up the BBQ sauce and pour over the ribs with just enough sauce to cover the ribs.