The rex is not especially hard to tame. There is several options in how to do it.

1 stand on a cliff that is higher then the rex an shoot it with tranqs of choice. It will start running when the torpor get high.

2. Use a thyla or wolf or equus or any other fast tame that you can use weapons when riding. Shoot the rex with some tranqs of choice and run away a bit. Shoot again the run away. Repeat untill the rex start to run away and then run after the rex and shoot till ita a sleep.

3. Use a Griffin and fly above it and shoot with tranqs. Fly after and shoot with tranqs till its asleep.

4. Have a tribe pick you up with an argy and fly above the rex and shoot tranqs and then fly after when it starts to run.

Hope it was helpful to someone 😊