It may control the tv volume by hdmi-cec, see if you can turn it on through the tv menu and refresh the devices attached to the tv. If it does control the volume, it may be able to turn on the tv by pressing the home key on the roku remote. But there is no way of turning the tv off.
Tv volume control via cec only worked on certain tv brands and models, I could never get it to work on the panasonic tv I owned at the time. Recently I did get an old roku 3 remote to control the volume on my TCL Roku tv.
You can get the voice remote for $20 at Walmart, Best Buy or even on-line at Amazon.