Poor customer service

Bought a device at mtn highveld mall, I was given a Sim that is not even registered on the mtn network. I went back to the store more than 3 times to get this sorted, I'm still sitting with the same problem. I even complained on the mtn website but still sitting with the same problem. This a huge inconvenience in my life because I had use for the Sim now my life is on a stand still because of someone's incompetence somewhere. Mtn doesn't take the customers seriously.