Selfhosted

Check if the router has the possibility to isolate the lan port. That way the port on the router can not talk to other devices in different ports or wlan.

Second possibility is to check if the router supports VLAN. If so you can put the TV or a port on a separate VLAN.

If all that is not possible, consider removing the cable and connect the tv wireless. That way you can put the tv on the guest WiFi network. That should come with isolation by default.

If you don’t want that either, you can resort to extra hardware. Any device with two lan ports could do. Make one port a dhcp based wan port connected to the current network and the other port goes to the tv. Run a dhcp server and nat and you have the tv isolated.

If you want to keep it wired then you'll need to put it on a separate VLAN from your other devices. A VLAN effectively allows you to create separate ethernet networks over the same physical network. We use them at work to keep factory hardware separate from office hardware and I use them at home to keep a vpn open for streaming geolocked content from another country. Traffic between the two VLANs has to be routed just like it would if they were separate physical networks.

I have an Edgerouter POE which has a small built in switch and supports VLANs so I can easily dedicate a port on the switch to a particular VLAN. In my case I route that traffic through wireguard, but in your case all you really need is setting up NAT for internet access and not route it with your other VLAN.

Any commercial grade routers support VLANs, i've seen it on unifi, aruba and fortigate and have never heard of it not being supported.

As others have pointed out, if you have a switch between your TV and Router then that'll need to be a managed switch that can trunk the vlan code back to the router, otherwise all the traffic will be comingled.

Other thoughts:

You might be able to arrange your IPs to sort of fake it. If your router is 192.168.1.1 and you make the TV be 192.168.1.2. Then you could give your TV a static IP configuration and tell it that it's subnet mask is 255.255.255.252. Then it'd only consider the IPs 192.168.1.1-192.168.1.2 as being in it's local network and if it tries to access something else on the LAN then it'll send it to the router for forwarding.

I'm not sure what your router would do in that situation, but it seems unlikely it'd manage to forward that packet. You'd have to avoid putting any device on 192.168.1.3 (as that'd be the routers broadcast address) but I think you could probably make that work. It's not really secure (as anyone that compromises the TV could change the subnet) and it'd still be possible for devices on your network to send UDP packets (but not get replies from) the TV. It's also not really extendable and you probably can't get a second TV to work like that (and definitely not three), but it wouldn't require switching to commercial routers.

I've got myself a second router and created a second wifi and lan with it. All my smart home devices are in there and also the tv.

You are probably a lot more technical than I am, but I would solve it by putting the TV on my guest network that comes out of the box of my mesh network…

How it's implemented can vary, but you're gonna take one of three approaches

• Microsegmentstion - On a home network this is the hardest but ensures there's no overlap
• Separate VLAN - this is usually good if your router can support it and have multiple gateways for each VLAN. Your router can then restrict traffic. Unifi gear does this well and I use this set up to segment my guest and IoT traffic
• Separate subnets - if your router doesn't support multiple VLANs this can work, but you still need a router that supports it

The latter two can actually work with an unmanaged switch as long as you tag your vlans correctly. The key is having a router than can handle it.