If the ISP charges you a monthly fee for the modem/router (you are "leasing" the modem), and you have been a long-term customer, you may have already paid the full price of the modem; yet you do not own it.

Let's say the customer connects their own Wifi router to the ISP modem/router via Ethernet cable. There is no need for the ISP modem/router's Wifi AP. Does the ISP modem/router allow the customer to disable it?