Something like a chromecast would be the easiest solution.

Me personally, I just like having a media pc hooked up to my tv. I bought an amazon fire tv cause it was fairly cheap for 4k and its never been hooked up to the internet.

Depends on your router. Some have the ability to disable internet access to single devices while leaving their internal network access intact.

You probably can give it a static ip through your router and block any access to the internet for it. Could even set up pihole to block the ads from coming in to any device. That said, it's possible the TV has built in ads or error messages to show in place of the ads when offline/blocked, or may just not even work if offline for longer than x minutes/hours/days

That's how I handled my "smart" TV - I had a spare minipc from my old homelab, threw Linux on it and plugged it into the HDMI port.

It has never connected to my network, despite the BestBuy employee insisting it would need firmware updates for better picture.

if it gives you the option you could try setting the DNS settings to something that doesn't exist.

Maybe if you give it its own vlan? Idk tho

You can probably get a DNS based ad blocker and configure your router DHCP to assign it to devices.

You may use it as a display and connect it to a Raspberry Pi (or other mini-PC) with Kodi on it.

...that's what I've been doing. I've accepted my fate

Sure, break it's routing. You can give it a fake DNS server (like a pihole that blocks everything), you could set up routing rules that block everything not addressed in the network ip range, there's a ton of ways I can think to do it off the top of my head. It might require some tinkering though