MTN router is a ****

I ordered a router online I received a call from the agent we made an agreement that they will deliver the router the agreement was to pay R250 per month. I made upfront payment of R391 they said it will cover november and december i will start paying for January. They delivered the router when i got home it does not have coverage I emailed them they asked me to go to the nearest mtn and give back the router

When I asked them about my refund they said my account was on arrears what arrears if i did not even use the router?