MTN Customer Support Failure

I signed a contract for MTN WiFi, but the service only work for a week since is limited Wi-Fi. When I tried to return it, the MTN store refused to accept it without a delivery note — because it was delivered and bought online. I was told to call *** and *** to request the delivery note, but for two weeks no one has answered. MTN keeps apologizing but offers no solution, leaving me stuck with a useless product while they continue to call demanding payments. This is unacceptable service and a clear failure of customer support, what can I do in this stressing situation, their bill is growing everyday.