These cheats need to be entered in Live mode.
Make sure testing cheats is turned on then copy and paste these into your cheats bar:
traits.equip_trait trait_Hidden_EP14World_NPC_MysteriousRancher
careers.add_career career_Adult_EP14World_NPC_MysteriousRancher
Remember that anytime you make the Sim active and load into their household, they will lose their NPC role. This also means going into CAS then exiting back into Live mode with them. So I always add the trait and career last, then exit to manage worlds, move the household to unplayed households and evict them from the lot. Some NPC roles do have other requirements like the base game fisherman. They can't be a young adult, only an adult or elder. Gym trainers need the active trait and max fitness skill. I don't know if the Mysterious Rancher has other requirements, but mine is an elder with the rancher, horse lover, and romantic traits. Make sure to evict them from their lot because they may need to be homeless.
I uploaded a screenshot showing that it does work. Mine shows up on time at the town center (behind him is the horse trainer I also made).
