Fidelity Security incompetence

Tried to cancel my account in November but I was shunted from pillar to post with no assistance from anyone. To add insult to injury my monthly payments were not entered into their records and they issued me with a letter of demand despite me having sent them proof of payment made every month. January 2024 I receive another letter of demand and threats of blacklisting . Not once have they acknowledged their error, the only response from their legal department was in January to their coastal care centre to assist with cancellation. January 14th I receive further accounts showing me in arrears. Good luck to anyone serviced ? by Fidelity (ADT / National security)