You only get a valid certificate for your router's address. But if you can take apart the router, you don't need to hijack the DNS, you can simply control its traffic.

But if you're a guest in my home and I see you take apart my router, you'll have to answer a few questions. Same in an office or coffeshop. Having LAN access doesn't mean you have complete physical control of the router. So the HTTPS is not useless.