Contract upgrade gone wrong

So here we go, last week I app**** for my upgrade online and everything went well but when I received my device I received the wrong item so I then proceeded to get hold of people at Mtn for well over an hour spoke to 6 different people and no one could help me because it was an online order and it was done through another company that Mtn decide to do sales on thier behalf so now I'm trying to solve this problem as I have the wrong device that I'm paying for and it's not what I ordered, at this point Mtn can take thier device and contract and go do what they want with it cause I'm not interested in dealing with this matter anymore iv tried to solve this and to no luck.