Mnt refusing to cancel my cellphone contract with them

Hello Peter I would like to lodge a complain about this service provider I have a cellphone contract that I finished paying off July 2022 since then I have been trying to cancel this contract till today without any luck from September until 1 December 2022 were I spoke to a lady who assisted and told it will be cancelled on the 31/12/2022 but again on the 28 January 2023 I received an invoice again that I must pay and I have been paying for something that am not even getting cause I no longer have that simcard I lost it before I even finishing to pay it on 26 March 2022 I even went to mnt store today at Vincent mall east london at Eastern cape 2 consultant I got from there they didn't help me they told me they can't assist me I must call 135 even after explaining to them my experience with that cancelation department that those ladies there are helpful at all. Please hello Peter assist this is my last resort I feel like mnt employees are ********** because the other lady that I got there when called she upgraded my contract without my consent instead of canceling it I had to again call to cancel that order that I didn't even request for