Cancellation notice not accepted

They have almost completed installing fiber. 29 July I login to my account as a client of Mweb with month to month internet. There is no link on the website to submitt cancellation of service so I look up previous e-mail addresses and forward a request to Mweb on 29 July 2019 to cancel. Happiness? No! Their accounts division which deals with cancellations aren't available over week-ends and only monday to friday between 8am(already started working 07h45) to 5pm (reach home just before 17h00 after work). Now they have issued me an invoice for Sept 2019, after I eventually get thru and give a mail address to send proof of request to cancel because the e-mail address is no longer valid. What I find strange is Sept is still coming, I've already paid my Aug 2019 account in full. They refuse to assist despite receiving a copy of my cancellation request 5 Aug 2019. 30 days notice then actually means the service should be terminated 5 Sept = 30 days but Mweb want to charge a full month, they have not and could not have paid for future data which has not been used. My Telkom ADSL line will be terminated 31 August 2019 so I cannot use internet or data even if I tried or wanted to. Is there anyone who can help or advise.