I am trying to use the Roku's ECP API, specifically the volume control commands (VolumeDown, VolumeMute, VolumeUp) to control a CEC device (Samsung TV) connected to a Roku Express 4K+.
I can send the command but I don't get any response and the TV certainly doesn't change its volume.
The Keypress section of the API docs (https://developer.roku.com/docs/developer-program/debugging/external-control-api.md) says that "Some Roku devices, such as Roku TVs, also support" those commands but it isn't very specific as to what works and what doesn't or how to know if it is failing (response, error code, etc.)
The DIAL protocol (see same page) doesn't seem to be an option. The Roku Android app uses that protocol I think and it doesn't have volume control for the 4K+ either.
Does anyone know if this is even possible?
Volume can only be controlled on a TV with Roku built in. You can not control volume on a connected Roku STB.
Some Roku devices, such as Roku TVs, also support:
Roku TV devices also support changing the channel when watching the TV tuner input: