Put three Dino gateways in a square but then there will be a gap. Put a LARGE bear trap in the middle, and have a fourth Dino gateway in your inventory. Lead the Rex into the trap and put the fourth gateway down. Then shoot the Rex with tranq arrows or darts. When it falls down give it food. If it’s torpor gets low then feed it narcotics or narcobarrys! Make sure you use STONE Dino gateways! I hope this helps!