Denied Upgrade

I have been with Cell C from the start of business. I am due for an upgrade and had a late payment in November due to me being unemployed for 4 months! Cell C states that I should not miss a payment for 4 months...this was my late payment, paid but late yet they deny me an upgrade! They should waiver the November late payment, paid fully and grant me the upgrade! I called their retention number 7 times this morning and what horrible service!!!! I will cancel my contract now. calling to cancel! no good service anymore, sadly