Disgusted MTN customer

I am utterly disappointed in the service I am getting. I am a MTN customer for over 15 years, and now there is loop holes being done when doing an upgrade. I upgrade in June when I was eligible for an upgrade to a sim/data only, because I have a device. The sales person said I would then pay my new contract fee of R129 in the following month. But from July till now I am paying over R500 for my device which I did not sign up for. I demand a refund for July - now. Please can someone assist me because i have been to MTN and have sent emails but no one is doing their work.