Hello, I have a Philips TV with Android TV built in, For whatever reason every time I turn on my TV it has disconnected from my WiFi and connected to a nearby open network. It is massively inconvenient to have to reconnect every time.
On Android there is an option to stop a phone from connected automatically to open networks, But I cant see such an option on Android TV!
Is your phone (or whatever you are streaming from) connected to that open network before you turn the tv on? The streaming device and the tv have to be connected to the same network to work -- so if your phone is already connected, it may be switching the tv as well.
Also, do you have the option to hardwire? If so, the tv has the option to turn on only when you use your streaming device (Wake On Cast) -- which might be slightly annoying, but might resolve the issue.
You should be able to "forget" the open network in the options.