Telkom mobile deducting payment after cancellation of contract

Had mobile data line for years using wifi at home, when fibre was installed in area requested Telkom to assist in switching to fibre, not willing to assist, waited 12months for contract to expire January 2020, cancelled data line with Telkom 03/01 with month's notice period, my line was disabled 03/02 as requested and could not use remaining data. On 05/02 Telkom deducted another payment from my bank account, upon query they could not provide a valid reason why another payment was deducted after cancellation?