Mantop-
Settings / Remotes & devices / Remotes / (click on remote with checkmark) / Set up remote for TV control / CEC setup -- then follow prompts.
If not successful, return and run IR setup -- and follow prompts.
EDITED TO ADD:
BUT even if CEC setup doesn't work for volume, mute, and power on your TV it may still support this CEC function:
Roku remotes cannot directly control the TV's input/source control. BUT, the TV itself may do some of this for you. To turn this feature on in your Roku, enable "1-touch play" via Roku menu path "Settings > System > Control other devices".
With CEC enabled on the TV, pressing a Roku control button on your Roku remote (not the power, volume, or mute buttons if you have them, because they act on the TV not the Roku) "One-touch play" should wake the TV from a standby state if necessary and switch the TV to the HDMI port to which your Roku is connected without you having to do it via the TV remote.