How to tame Rex:

Get a bird and find a Rex close to a small cliff.

Clear out all other hostile Dinos.

Lure Rex to bottom of cliff with bird.

Dismount bird and shoot tranq arrows at Rex.

When it runs away, hop on bird and dismount within shooting distance and continue shooting until it’s unconscious.

You got a Rex! Worked for me, and now I have 2 29 level rexes.