Lack of communication

I had a contract with MTN for airtime & data service for a year,31 January 2023 was my last month with them, they deducted R616.36.On the 31st of August 2023 I started to receive messages from them that I owe R1800 which is cancelation fee's.My question is arrears for what because their Sim cards from the 1st of February they were not working I communicat with them using my Vodacom Sim,and why they didn't inform me about cancelation fees from February?