LTE Month to Month

Good day, please assist me with Mtn, I opened Month to Month LTE with them which worked for 2 days from 29th August to 1st September 2 days precisely, i took router back to store 3rd September 2023 and I cancelled my service, they went to bill me on top of subscription i paid with no service rendered to me. I am now stuck with bills that Mtn are charging me on something I don't even own or used for 7 days.i fought with them in store and social media but i am not getting help. How can i be billed on service that I cancelled on their own store. If the consultant make mistake on cancellation why do i have to suffer as the customer. Can i please get my refund and my name out of Mtn it's causing negative impact on my credit scores and can i get my refund too