I Need Help! I returned the wifi box and still got charged $400!

I had service issues so they sent me a new box. By the time the box arrived my service was working again. I returned the new wifi box and I was still charged $400. I called and they gave me a refund. Next thing I know my service was suspended for unpaid bill. They have now restored my service. However, they are not removing the $400 from bill. I'm stuck in a loop now of not paying this and getting my service suspended. Please help! They have the wifi box back. I need a $400 refund.