Firstly check in your router settings if you've got some unauthorized IPs using your wi-fi. Secondly your best bet would be call the ISP's support, they'll identify and fix the problem faster than we would.
I had a DSL connection at my previous home and my bandwidth speed used to fall drastically like every 2 years. Then I called the ISP, they sent me people to change my router with a newer one and speed was back as usual. That happened like 3 or 4 times. Only once did the cable age (it was some 14 years old) and then the connection was cut entirely.