MTN doesn't keep their promises and treat customers unfairly

I app**** for MTN data contract online via Mondo and the lady who was assisting me said I will be able to port the Sim as soon as I receive the contract Sim card as I wanted to keep my number. On Thursday I received the Sim then Friday I went to MTN store to Port but when I get there the Consultant informed me that I won't be able to port contract Sim Card that means the lady from Mondo **** to me to make a sale. I then immediately asked the Consultant to cancel the contract as I don't want it anymore. I followed up the following week to MTN to confirm the cancelation and I was told it is canceled. Remember I haven't used the Sim at all come month end MTN deduct Money from my acou account, I then call them and ask why was that they told me it was not supposed to deduct as the Contract is canceled. That was end of June I followed up with Mondo again and they said they escalated the matter with MTN the money will be returned I month went by and did not receive cash back I then called MTN and they said because I don't ha e contract I can reverse the money myself and I asked if that will not affect my Credit portfolio and he said no because I don't have contact with MTN. I then received an SMS from MTN saying I owe them I then asked Mondo what's really happening and they promised to call me back regarding the matter as MTN is not suppose to deduct cos I don't have contact. Yesterday 31.07.2021 MTN attempted to deduct from my account again. Please assist me with the matter as I have not used that Sim of theirs.