I could not find any capability for controlling TV brightness. I also could not find any information on the documentation about this. Is controlling the brightness not implemented or am I missing something? As this is a somehow essential feature I would be very surprised if this is not implemented as a service…
The available capabilities for any device depend on the device manufacturer and the ones that created the integration.
There’s no documentation about how to control specific device models because each has differences, have you checked if there’s a technical sheet where you can find if the device accepts that?
If you have already checked the device’s list of capabilities in the API and there wasn’t any to help you, then it’s difficult to know if you could send OCF commands to it as mentioned here: REST API for display/light on Samsung Windfree AC - #8 by JRFabbi
or here: HW-Q990B/Q995B: The app can control the sound mode, but I cannot find it in Smart Things capabilities - #11 by S3basuchian
Something interesting about OCF devices like TVs, Washing machines, ACs, etc. is that they work on the plugin level (app), not the API level, so not all commands will work as expected when sent from the API.
Do you have an access token and can do a simple GET to your device information? If YES you can check what you receive on ‘execute’ status after you change this option on SmartThings APP, is a best way to debug your device and get some info on how to send execute commands (OCF) to your device.