To make the rub: In a large saute pan over med-high heat, toast the cumin seeds, chili powder, paprika, mustard seeds, and coriander seeds, shaking the pan occasionally, until the spices start to smoke, about 2-3 minutes.
Transfer mixture to a mortar or coffee grinder; add in salt, brown sugar, garlic salt, and cayenne; grind the mixture coarsely.
Trim nearly all of the fat from the prime rib.
Evenly spread the rub all over the prime rib.
Cover with plastic wrap and refrigerate for at least 3 hours and as long as 12 hours.
Grill the prime rib, fat side up, INDIRECTLY over medium heat for about 2 ½ hours.
Remove the prime rib when it reaches the desired doneness (use an internal thermometer, if desired).
Loosely cover the prime rib with foil and allow it to rest for 30 minutes before slicing (internal temperature will rise 5-10 degrees).
Cut into slices ½ to 1 inch thick; serve warm.