I honestly don't know of anyone who brines ribs, but that doesn't mean you can't give it a try...lots of different ways to skin a cat. I think you already hit on the simple solution: If your ribs were not tender, they needed to cook longer. Remember that cook times that are posted and reposted are only general guidelines...every hunk of meat will be different. There are several ways to tell when a rack of ribs are done to your desired tenderness. My 'go to' methods are the bend test and the toothpick test.
Bend test: use a pair of BBQ tongs to pick up the rack near the middle, allowing the rack to bend in the middle. When the rack bends easily to near 90* and the rib meat on the top of the rack starts to crack or tear as the rack bends, they are done
Toothpick or probe test: take a toothpick, a kabob skewer, or a sharp probe...poke the rib meat in multiple places across the rack. When the toothpick glides in and out with little resistance, they're done.