Bad expierce

My contract expired in May 2022. Mtn never notified me. End of June double the normal amount went of through a debit order. I contacted mtn and asked them what is going on. They said because my contract ended May I was not eligible for the promotion anymore and was paying full amount. I asked them as my contract has ended a month prior to end my contract as I have served the full term of my contract. They agreed. End of July they debited me again and now end of August again. I live in another country so it is very difficult for me to go in store and sort this mess out. They promised me end of June would be my last debit