Overcharged!!

So I have a contract with the service provider. I paid my bill for the end of June 5 days before debit date. All because I've changed banks however I couldn't get ahold of telkom to facilitate the changes hence I made the manual payment. I checked before the 25th and they reflected on their app. Telkom tried to debit my account on the 26th, and again on the 29th. I've accured bank charges over R300 for that, I just received my invoice for the month, I've been charged a rejection fee twice for failed Debits but the bill was paid on the 18th and reflected on the app on the 21st June. I want help. I want to cancel this **** contract