Contract ended in march 2024

I had a contract with ADT Security and I experienced the problem with the device .I ask them to come and help me and they did but my problem was I have to buy a new device .How can I buy a new device while I pay monthly installment? I ask them why they did not install a new device and they said it is not according to their policy. I told them that I cancel my contract with them in 2023 and they said the contract will end in march 2024. I was waiting for march 2024 and the month of march arrived. Now they send me sms and e-mails that I did not pay for their services. I responded by saying that as per our conversation, the contract is ended. I am very disappointed with ADT as their service is very poor.