In my country, the ISP rents you a modem and router. I told them I had my own modem and router during setup and my monthly cost is slightly less than their advertised price.
I am fortunate that my ISP gives me a routable address, but it is still only dynamic and may change a couple times a year. I would have to pay for a commercial plan if I want a static IP. Some other local ISPs use carrier grade NAT, but you can still request a publicly routable static IP with a business plan. Maybe you can ask your ISP for that?