Mnt does not honor its own contract terms

I have a contract with mtn which dates back more than 8 years now. My phone was blocked on Friday. When I called mtn the person I spoke to said that its because I owe subscription for May 2020. I asked how I can owe for so long and they only block me now & its a debit set up I have with mtn. How can they wait 11 months then tell me that am owing them when they are the ones debiting from my account? Also in March they tried to debit on the 18th but our contract states that they must debit on the last day of the month. If they are using an automated system as they claim then why do debits happen at any time and not on the agreed time between the 2 parties? Isn't that a breach of contract? And it means that my bank account is not safe as mtn can debit at any moment they feel like. Why then sign a contract if mtn is going to do as they please disregarding the terms of our contract? I am not happy about how they do business. From my knowlegde they must block me the same month I owe them if I owe them not 11 months after