Step 1Toast the cumin and fennel seeds in a small skillet over low heat until lightly browned and fragrant, 2 to 3 minutes. Remove from heat and cool to room temperature. Grind the seeds with a mortar and pestle, then combine in a small bowl with the ancho and chipotle chile peppers, the sugar, dry mustard, garlic powder, 2 teaspoons of salt and cayenne pepper. Set aside.
Step 2Rub the meaty surfaces of the ribs with the cut cloves of garlic. Season the ribs with salt to taste. Sprinkle the spice mixture over ribs and rub onto all surfaces of the ribs. Cover and let stand 30 minutes.
Step 3Heat an indoor or outdoor grill to medium-high heat. Grill the ribs, in batches if necessary, until browned and crusty, about 15 minutes for medium to medium-rare. Remove the ribs to a serving platter and squeeze lime juice over the top.