Cellphone contract

Very disappointed at mtn. I took a iphone 13 pro on contract on the 24th of June and realized I need iphone 13 Promax instead. I called customer care and was advise they can't help me because I purchased phone at the store and not online. I went back to store and they refused to help me to swop to iphone pro max obviously inconjuction with new deal. I was told to cancel contract and pay cancelation fee which is the value of phone. I don't need to cancel. I just need iphone 13 pro max. Manager advised me I have already signed on the dotted line and I am stuck with phone I don't even like. Mtn is this how you treat you loyal client. I feel very *****ed over with no help. Customer service is bad