MTN bad service

I took router and Sim on contract. I hardly use it but when I do that's when I put it on. Less than 6 months the battery of the router got swollen. Went to MTN outlets no one could help me. I decided let me rather cancel it because I will charged for something I'm not using. I sent emails Jan 2023, no response, it's March 2023 still no one has contacted me instead they send me invoices.