Debit order dispute and reversal

Today I had an amount deducted from my account:

31 Mar 2020 TELKOMMOBI*** 459.00

This is in error as the Telkom account is the process of being cancelled. I would like to dispute/reverse this debit order, as any debit order can be disputed within 30 days on the account, as per legislation and law. However, the online banking application does not allow this. I am not sure how to proceed. I know I can go to Telkom an attempt this as well (but so far has not been met with much assistance); I am also allowed to impede any funds being deducted. I would like to choose this route.

Regards, Werner