Mtn overpaying

I bought a router in feb but the router gave me problem of not connecting to internet and i would call mtn and it works for that day and the following day it will not work it continued until i said its not working for me since i am paying for services i am not using then i talked to them over the phone that i want to cancel as its not working and only works once i talk to them then they said i should go to my nearest store i went there and they told me since i ordered online i should call for them to pick it up i calles but they said they dont pick up they only collect at mtn store i went back this time on the 17th of march and they cancelled but said it might bill you for next month which was gonna be R260 before it cancels i said ok but now i receive a statement saying i should pay R500 i am billed even for April for services i am no longer using and the device is with them and i dont have that money since i was told they will bill for 1 month and its not even cancelled yet