For what it’s worth, I just bought a TCL 55S450F (55 inch 4K HDR FireTV) specifically because it does not ever need an internet connection to function (expressly stated in the manual). It is currently on Amazon for $268 (they have other sizes at other prices). It’s a great TV, considering the price. The only real drawback for me is the remote is Bluetooth, rather than infrared (less compatible with universal remotes).
Note that for full dumb TV effect, you’ll want to go into the settings and tell it to resume the last input, rather than going to the home screen when you turn it on (without connecting it to the Internet, the home screen is basically just a big banner telling you it’s not connected, and when you dismiss that, it just allows you to access inputs and manage settings).