Online MTN cell phone contract via Mondo

I was sold a cell phone contract for MTN by Mondo, was assured the phone would suit my requirements, it did not by any means. I returned it to MTN unopened 4 days later. I am still being charged for it, despite monthly visits to MTN, emails to Mondo & MTN and weekly phone calls. My MTN account is suspended monthly if I don't pay. This was from January 2021