EXTREMELY BAD SERVICE

I have app**** for fibre and for approx 6 weeks we were told that our fibre is all installed and working!! STILL NOT WORKING!!!!! I want to cancel as I can't go along with people who's service is so poor, but now they want to charge a cancellation fee. HOW ON EARTH DO YOU CHARGE A CANCELLATION FEE IF YOU ARE THE ONES THAT CAN'T DELIVER A SERVICE AS PROMISED?????!!!!!