Utterly disappointed in Cell C

I received my April Statement and the Data contract's billing was off by R265.48, which totaled it to R725.29 instead of R522.28 - It seemed that my discount had been incorrectly calculated. (Contract is on month 21 of 24) I made contact with regards to the billing on my statement on the following dates:

* 11 April 2021 - I logged a query with regards to my billing * 15 April 2021 - Irene called to inform me that the "Back Office/Finance Department will be in contact in 2 days time with resolution of my query. * 20 April 2021 - I phoned again to hear why I hadn't received any feedback from above mentioned office * 27 April 2021 - I received ONE missed call at 13:06 (Public Holiday) * 28 April 2021 - I called again to enquire about the resolution of my query * 1 May 2021 - phoned to cancel my contract as the incorrect amount was debited off my account. Agent unable to cancel, must phone back on Monday when offices re-open. 3 May 2021 - Consultant informs me that I must PAY the penalty fee of R141 first before she is allowed to cancel my contract. I should also send my POP to the Proof of payments department before I contact Cell C to cancel. *6 May 2021 - Proof of payment reflects and I am able to actively cancel the contract.

6 May 2021 (after cancellation) I receive the following reply on email:

"Thank you for contacting Cell C Please be advised that the discount was loaded early on the 17.04.2019 therefore it ended on the 17.04.2021 however if you which to continue getting the discount you may upgrade to a new plan that offers discount on the number 084143 option 1"

To my knowledge my contract or benefit has not changed or been amended and I have received no notification that it has or would.

My question is shouldn't the benefit apply for the FULL duration of the agreed contract?