Terrible service

What a poor service. My contract has expired in May 2024. However I phoned MTN around the 11 /12 June 2024 to cancel my contract. I was told to pay June bill and the last payment will be July 2024(serving 30 days notice) . I made a payment on the 21 June 2024. To my surprise today when I visited MTN shop to make July payment, the official informed me that as my contract cancelation was only finalised on the 14/07/2024. I have to make another payment in August. I personally made a 30 notice and it's not my fault for my contract to only be finalised on the 14/07/2024. They need to sort it out.