Rex's tend to get stuck on rocks easily, so what you should do is find a valley like spot. (( all on one side and then another on the opposite side) then what you want to do is use a bird to find a way up to the top of the cliff line, start pounding tranqs. Onice it drops, your then have a wall (unbreakable) and then put spikes around the Rex for extra protection.
I recommend using a scorpion to tame it, pump all levels into health on the scorpion. Then just attack the Rex but with 10 to 20 seconds in between hits, DO NOT pump anything into dm. Also this is what I use to tame literally everything, and it doesn’t even cost anything. Pls put this up in chat for everyone to go “oh that’s not a bad idea” 👌😄👍
Just tamed one I will tell you my experience, so I found one and shot it with tranc DARTS (but tranc arrows work just as well although you need a few more :)) and it kept on getting deststracted by other Dino's which made it not attack me, so after I knocd it out I killed the other animals attacking the unconchis body with my Pump shotgun and gave it raw meat and a couple of bio-toxins and boom I got Rexy I hoped this helps on your next Rex tame