Not tender at all

Sounds like they needed more time (as evidenced by your solution!). Tough cuts of meat like that need lots of long low cooking to convert the collagen in the connective tissue to gelatin, giving them that "fall apart" consistency. The thing is though, when cooking, the muscle fibers contract. Once that gelatin conversion happens, you don't notice that, but until that happens your meat will be tough.

Baby backs are favorites and here's how I make them.
Oven 350
-Slab of ribs (underside membrane removed)
-Glass baking dish
-Sprinkle both sides with onion powder and sea salt (lightly with the salt)
-Bone side down
-Cover tightly with foil NO liquid needed
-After 1-1/2 hours remove foil
-Baste with Sweet Baby Rays sauce on both side
-Return to oven for 20 minutes baste meat side with more sauce
-Roast another 20 minutes...
-Optional turn on broiler for 5 minutes to get char on ribs
Fall off the bone delicious!
BB💐

I normally make a foil packet for ribs and cook them or 4 hours. I use a spice rub and pour about 1 cup of stock and a little bit of liquid smoke inside the packet before sealing it up. Then at the end, I put barbecue sauce over them and broil the ribs for a few minutes.

I ended up adding a cup of apple juice and putting the foil back on for another hour. They got tender and I basted with the liquid in the pan and put back in the over at 350 for 30 minutes. They turned out great.