I tamed one at lvl 45 and its really easy

Step 1:

Get biotoxin (preferably) or narcotics, maybe also narcoberries

Step 2: Get raw meat, prime meat, or mutton

Step 3: Get a bow, crossbow, or longneck.

Tranq arrows for bow and crossbow, tranq darts for longneck or other rifles

Step 4: Find rex

Step 5: Get on rock and bring and shoot (bring a fast or flying tame just in case, its optional)

Step 6: Once it’s knocked out, feed it the meat that I preferred

Step 7: Feed it, wait, and feed it toxins or narcotics when unconscious level is low

Step 8: Wait.

Step 9: Once you have tamed it, congrats! You have one of the most powerful non-boss creatures!

Extra: It’s saddle unlocks at lvl 74