Mtn mondo worse ever

Hi I have had a nightmare by accepting a contract from mondo I bought a laptop and they sent me an incorrect one so I called them and sent it back as I couldn't hand it to mtn itself as it wasn't a receiving store the same couiers took it back however they didn't do a full credit of that laptop to mtn they missing a second credit as I was told I have sent 100s of rands in calls only to be told it would be sorted a year later I find out I have been negative on my credit bureau due to this mtn bill it's a absolute nightmare as I needed a loan and was declined due to mtn this has cost me to spitlral downwards financially please can I get help I don't know what to do my last help was given wen I was told it would be sorted out the next day Ref rco298122