Vodacom wifi contract

I regret taking Vodacom 3 years wifi contract. I have set a debit date for 15th of every month, but if the 15th fall on a weekend they choose not to debit and debit me double the following month. Worse part when they didn't debit it reflect on my credit report as a none payment. The same thing happened 3 times now. Also when they are increasing the amount they just increase without notifying me.