It is hard to tame rex due to having a partner or other creatures that might aggro on you or the rex itself. Especially the rex itself will aggro on the creature too. So all you need is a rideable creature to run around it and shoot tranq arrows with a cross bow and feed it prime or mutton if you can get a hold of a ovis.
Pretty easy to tame as long as you are prepared. Always bring extra tranqs and narcotics to be on the safe side just because it takes a lot to bring them down. Upgraded rifles and crossbows are a godsend and make it even easier too. Get a Pteranodon or Argentavis, find a cliff/high ground where the Rex can’t get you and make it rain with darts and arrows. And when they start to flee once their torpor increases, run it down and don’t let up. I don’t make/use traps but this tactic works well for me. The giant murder lizard will be yours soon enough! Happy hunting and good luck!
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.
So everyone is saying to build traps and go on birds but if your lower level looking for a very low level one to tame 0 narcos needed and if it’s low level you could just go on a big rock or ledge and shoot it with tranqs that’s what I did with my rexs