@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.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4850 | Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X