MTN router

My home router contract ended on the 16 Nov 2025 the beginning of Nov I went to the branch to cancel they couldn't and said I can cancel after 30 days to my surprise they billed me even more than what I was paying. I paid 514 but I was billed 668 without explanation I sent several emails regarding cancelation but up to date they didn't cancel but billed me.very upset and need my refund of 668. They know when to sent an invoice but when you called regarding network problem they don't attend to when you cancel they ask why do you cancel even though you explain to them that you have a network challenge which they don't attend to