I do think it should be illegal for ISPs to force a specific router. The routers they use are likely garbage and might not suit your needs depending on who you are, furthermore, knowing that several ISPs just had a pretty massive breach, I wouldnt trust ISP provided routers for a second considering the complex management tools they have access to with routers they supply. FloatArtifact has a good idea of cloning the ISP supplied router's mac address, though depending on your ISP, they may have more advanced ways of telling if your router is the one they gave you, like a heartbeat of some sort, as opposed to a basic MAC address check.