Router return and contract cancellation

I cancelled my month-to-month subscription and returned the router in another branch because I relocated and now i keep on getting an invoice for R2500 every month, customer service fail to help me understand why i owe so much money and when i go the store i returned the router, i’m being told the reason why is because i didn’t return the router at the store i bought it from, of which it could not be possible because my circumstances could not allow. I’m very disappointed with their service and i don’t think i’ll ever be a customer there again or even recommend someone to sign anything with them.