Double billing and refusing to pay back the extra billed amount

I have used LTE because the area where I stay did not have Fibre. As soon as fibre were installed I cancelled the LTE and They have installed fibre. For more than a year they are double billing me for the LTE as well as the fibre, Upon my enquiry about this they refused to pay back the double billing because they "can not find a cancellation request" Are you stupid - I can only use one connection and you installed it and upon request for the fibre product the LTE were cancelled. Yet they refuse to pay back the double billing