To tame rexes make a u shape with stone gate ways and place a behemoth gate way at the top like this

behemoth gate with door 👇

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nrml Gateways

Lead it in and while it's turning around inside shut the behemoth gate door behind it then it's trapped,

Then proceed to shoot the nap time arrows or darts at it till it is asleep and feed it kibble or prime meat till tamed,

If your taming with prime meat wait for it to starve a lot then feed prime meat after so it eats it all at once and if you need prime meat you can kill baby dinos for easy prime meat