BAD SERVICE FROM VODACOM FIBRE

I had a 100bps home fibre on a month to month at R879. On 4th March I contacted Vodacom fibre on WhatsApp to migrate to 50bps for R669 per month. On 26 March an amount of R899 was debited to my account. When I enquired I was told my enquiry came late for the change of debit order amount. I have just received an invoice of R1301. I called the call centre was told by the lady who assisted me that she would call me back. Now I called back and have been on hold for 6 whole minutes. Why must I pay more for 50bps than what I was paying for R100bps?