Just a random question/thought that popped up in my head: If you had one router with its real mac address connected to the internet on a particular ISP, and you cloned the Mac of another router to make it the same of the 1st one and connected it to the same ISP, would it make the connections for both or just one connection unstable, not work at all or would the ISP-level routing work around it?

As far as I’m aware ISPs (at least Virgin) lease routers their IP address based on their MAC. Essentially the ISP will be trying to lease the same IP to 2 devices at once.