Frustrated MTN customer

I took out a data contract with MTN as server through the iStore at Tygervalley in March 2019. I do not wish to renew this contract and was informed that the date of contract completion is 25/3/2021. I was told by both iStore and MTN stores at Tygervalley that I cannot cancel this contract I store. I was given an email address by a ‘service champion’ at Tygervalley for their retentions department on the 8th April and have forwarded the request for cancellation. I was informed by an automated response that cancellations cannot be done via email but needed to call ‘135’ and ask for the retentions department. I have been trying to contact them daily without any success. They told me at the MTN store on Monday that their lines were down. Today I have tried calling them several times and have had no success. How do I proceed at this point? I have been an MTN customer for nearly three decades but this experience is as bad as my attempt to cancel my Telkom contract which took 9 months !