Consumer rights revoked by Mtn with empty promises and under trained consultants

So on Saturday my wife surprised me with a brand new contract at mtn she took me in and let me describe what I expect from a phone and the sales consultant recommended me a phone for the specifications that I stated . So I was recommended the Samsung A30s 4gb Ram and 128 GB built in storage Mid range phone but still and excellent phone. Was promised by the Mtn representative at the time that this device is what I'm looking for so obviously I took the contract. So i get home open my new phone and start to test it out. So in short what my specifications were, was to be able to perform very well with the top 2 world grossing game Pubg mobile . Players unknown battlegrounds. So basically phone is a very latest release and hasn't gotten any reviews or bug encounters up until now. So while playing my well supported mobile game that is even locally known. I encountered that there is a big while touching more then 1 area on the touch screen which is known as a Multi touch delay bug. I could be wrong but anyway when multi touching the screen and sliding ur fingers up and down or left and right while touching more then one area of the screen it has a Delay of 0.5 sec response. Which is obviously a bug that has not been detected. But by me. So in this event I cannot use the phone for it is unable to do what I need it to do without fault. So if contacted the branch manager and even head office of mtn and it's been said that I've been revoked of my consumer rights and that consumer rights only apply with online purchases... ye I'm shocked too. Iv spoken to 2 different branch managers of mtn and they have told me that I do have consumer rights and that if I find a fault in the phone within 7 days i have the right to request for a cancellation. Yet the mtn branch at Clear water mall has been avoiding me and not answering my calls anybody please help this is a serious matter I even offered to pay more in my contract for a different phone that I know personally will work because of past encounters with the device I know will work. Oh and Btw my old phone that iv been using for more then 2 years The Samsung J7 prime 3 GB ram and 16 g built in does not have this issue yet it's a low range phone compared to a mid range phone.