Combine salt, black pepper, mustard and garlic in a bowl. Evenly rub the seasoning all over the outside of the roast.
Once your grill is preheated, place the roast on the grates, ensuring the ribs are facing the back end of the grill. Once the roast is placed on the grill, shut the lid to the grill.
After 45 minutes, lower the temperature of the grill to 325 degrees F. Cook for an additional 2.5 hours or until the internal temperature reaches 125 F. Remove the roast, letting it rest for about 15 minutes. Slice and enjoy!