Submerge the cedar planks in a 9x13 (22 x 33 cm) glass baking dish filled with water. Soak for at least 1 hour to prevent burning while grilling.
In a small bowl, whisk together the mustard, brown sugar, orange zest, salt, pepper, coriander, and cumin.
On a cutting board, pat the salmon fillets with paper towels to remove any excess moisture. Rub the mustard mixture evenly over the salmon.
Preheat the grill to 350°F (180°C).
Place the cedar planks on the grill over indirect heat, arrange the salmon fillets on top, and close the lid. Grill for 15-17 minutes, until the salmon is pink in the center and easily flakes with a fork.