Daylight robbing from mtn

Very bad service from mtn. I return my home router for cancelation of contract as I was on a month to month contract. Contract was set to be cancelled on 19.01.2023. In February they debited my account but there was no funds available. Now they telling me I owe them R2499 for the router that I returned it to the branch. Had to go back to the branch last week as they had to waive the fee of R2499 as the router was returned which it did and they gave me a reference number. I have all the relevant information and references number to all correspondence from mtn but no once can help me. How can I pay for something that I returned and it was in working condition and was assess by my. The lady in retention department by the name on yonela nodiwa that assisted me even gave me her email adress to mail her once i was at the branch ,I sent her 3 emails and no response, call centre sends me up and down as no one wants to deal with the query I'll won't pay for something I returned if no one can assist me I'll gave no other choice but to go to the ombudsman for help