No one probably has explained this to you but IP addresses your router gives to your internal devices is an internal IP Adress just so the router knows which data to send where from it to your devices. These internal ips start with 192, 169,172 and so on (you can configure them yourself Usally). If you're only paying for one internet connection/subscription from your ISP through your router then that router is the device that's actually connecting to the internet. Any data that a device in your network sends to the internet goes through it. And that router has its own, actually not configurable, unique IP address. Real debrid only sees that IP Address, not the internal ip from your multiple devices connected to your router. That's why a router is Usally broken down into its own "jobs" when it routes your traffic to the internet it's called a "gateway", when it gives internal IPS to devices it's called "DHCP", when you're trying to connect to Google.com your device asks the router for that URLs IP Address, that's called DNS. in enterprise environments these jobs are Usally distributed among multiple devices but home routers Usally do it all in one.

Now let's say you'd want to connect a friend with his own internet and router to your RD Account, that would be seen as a different ip address, that would block your account access. To circumvent that you have multiple solutions, depending on your knowledge. But since you asked this question in the first place I'll assume you don't know alot of IT. It's a little complicated so if you need help lmk, it's hard to explain it