MTN endless nightmare

I have been a customer of MTN for 15 years. I have 2 contracts. In October 23, I received a call from a call center, informing me that me that my Contracts were due to be upgraded. Because I was paying over R1000 for 2 contracts, I asked the lady for some cheaper options. I ended up upgrading one contract and asked if the other contract could be on a "month to month" basis to which she rep**** "no problem". At the end of October I was debited R660 which was great, I was spending less money for less data. Then, at the end of November I was debited R 2100.00, shocked and confused I called MTN to see what the problem was. I was informed that because I went onto a month to month, that my contract went from R330 to R1500 a month, just for one contract. I tried to tell them that I was never informed about this and never agreed to pay this amount. I went to my bank to reverse the debit order, MTN then disconnected both my contracts. After many, many calls and transfers, and holding, nobody from MTN could ever help me. After no assistance from the call center I went to the MTN store at coastlands in Umhlanga where I was told there is nothing they can do and I HAVE to pay the money. I told them that I cannot afford to pay that amount at once, so they set up a payment plan. R860 X 3 installments. My first installment was paid on the 29th December, and on the 1st January they then tried to debit R2500 from my account. I have now been cut off again, and still nobody can help me. When I phoned them today, they told me I need to make a new payment plan which will now be R 1000 x 3 installments when I've already paid my first installment that was agreed upon in store. Why is MTN treating loyal customers like this? How can my contact go from R330 to R1500 pm just for being month to month? Why was I never informed about the new amount when upgrading? I feel completely helpless, with no resolution and no data.