Price increase and incorrect billing amount.

I have had vodacom home wifi for the past 3 years. The contract ended in September 2025. I decided to cancel the. Month to month in December 2025. I had been paying R296. 48 monthly and when I confirmed the cancelation I was told that there will be no cancelation fees just a 30 day waiting period as I terminated after the 7th of the month. My debit order date passed on the 27 and a debit order of R500 went off on my account for the month of December. Today 7 January I received and invoice of R600 for the month of January. From my understanding I should not be paying cancelation fees and the new increased rates on my plan will on be effective from 1 February 2026. The email I received further stated that the plan I'm on will move from R296. 48 to R323.13. Why was I billed R600.