If you find one near a mountain get on the mountain and it will just look at you (it might try to climb up if it’s a small mountain/Hill ) then just shoot it in the face with dart tranq arrows etc ( just be careful of other dinos that may be on that hill or maintain with you )
If you use the bear trap strategy which is if you lead it into a Dino gateway that is shaped in a triangle and with a bear trap in the middle (Large Beartrap) and when it is stuck you must put a dinogate behind it to trap it and then knock out.
The Rex are very slow . Put a stone gates around him while he is stuck and use a bear trap then put the x3-x4 stone gates then tranq him out . Make sure you clear the area before setting the bear traps others creature can spring it and u would be left with using your self or a tanky dino for a distraction