Failing to reverse wrong upgrade made by mondo

I received poor service from mondo which made me to end up paying for devices that I don't have, which I have returned to them due to delivering the wrong order. I called them before the activation of contract informing them about it, and they told me to return the devices to nearest MTN which I did. But they continued with the activation of the contract and made me to pay for those devices I have returned. I have sent them several email and made several call to them to request them to fix the error they creates but with no help but empty promise. But month end they debit my account. Which I ended up reversing them because I can't continue paying for the devices I don't have. Never buy online products from mondo.