Would replacing the ISP provided router with a router that I buy work?

So my ISP (I am from Nepal) provides a fiber router of their own during installation, but they don't give me any access to the router through my computer.

If I replace that router with my own and connect the fiber cable to it, would it work? And would I get to access that router?

Note that I already have an additional router connected to the router through a LAN cable, but I want to replace the ISP provided router itself.