Router control

We have taken a router contract with MTN 8 months ago and discovered that network coverage at my residence is none existent. After all the struggles and countless calls to mtn we then took the router back to wonderpark mtn store within 7days of the existing contract and they took the router with the simcard back and told us that they going to cancel the contract

Now we getting calls from debt collectors telling us that mtn handed us over that we owe mtn so clearly they didn't do their job and when we visit the store they telling us that we have to speak to the accounts team.