MTN contract upgrade or downgrade?

So I have an MTN contract for 10 years now....and was due for an upgrade in December 2017. In December I received a call from someone working at Summersun Trading on behalf of MTN. Long story short I agree to an upgrade and should have received a Samsung J5 via courier within 7 days. Which I never did. So I call an ask and the person I spoke to says they will be sending it. And I never received the phone. Did not sign anything. So I assumed the upgrade did not happen. Yesterday I went to MTN to do an upgrade and was told but I already upgraded in December 2017. When I explained my situation I was told to phone 083 135 which I did and asked for the upgrade to be reversed. I was told by the lady I spoke to that she would send an email and it would take 24hrs for the matter to be attended to. Today I went back to MTN to check ..and still nothing. The manager of the store checked for me and then said Summersun trading is a very infamous company who has had many customers irate because they deliver devices late or the incorrect devices. When I asked for contact details for Summersun...no one had any..when I asked if I could cancel or stop the contract...I was told only Summersun could do it on their side...so imagine my frustration right now. The contract is with MTN yet they are pleading no knowledge of this company and its contact details. I don't have the phone and I have been paying for it for 8 months. And now no one can help me to cancel or stop the contract. What am I supposed to do now?