When you connect a new device to a ‘smart’ tv, you must pay homage to the manufacturer with a ritualistic dance. Plugging and unplugging the device. Turning them on and off in the correct sequence like entering a konami code.
Every time you want to switch devices, the tv must scan for them. And god forbid you lose power, or unplug something. You are granted the delight experience of doing it all over again.
I have fond memories of the days of just plugging something in, and pressing the input button. Instant gratification. It was a simpler time.
What is some other tech that used to be better?
There should be a section in the configuration about dhcp, which is how ipv4 addresses are given out on your network. What happens is when a device first connects to the network, it sends out a broadcast with its mac address - the dhcp server (in this case, your router/firewall) hears this, and sends back a reply allocating an address. You should be able to see a list of currently allocated addresses, and hopefully configure reservations to make those allocations permanent. To reserve an ipv4 address for a specific device, you need that device’s mac address.
Each item on that current allocations list should have a hostname, a mac address, and an ipv4 address. If it’s not clear by the hostname which device is the tv, you can look up each mac address and deduce from there (the first part of each address is unique to a specific manufacturer).
Once you have an ipv4 address reserved for the tv, you can set your outbound firewall rule to block it.
Ipv6, as I mentioned, is much more complicated. It might be possible to disable it completely on your router, and that’s likely the only way to block the tv from using it, but then your whole network will lose ipv6 capability across that boundary (probably not a lot of downside to that, though).
Good luck!