Complaint about mtn

I got a call from mtn offering me about a wifi home router n I was interested n apply for it the person said is unlimited they then deliver it to my work when I get home I connected it n the was no coverage I call mtn customer care n they said I must apply for return I did n they told me to return it at the store when I go to the store they tell me to return it online... I tried all my best to return it but they didn't take it the most boring part is that I'm now owing 7000 n I'm even blacklisted because of it.... I'm writing this because I need help