If you want to press a button on your Roku remote to automatically wake up your Roku device and switch to the HDMI input that it is currently plugged into, you will need to verify that both "1-touch play" is enabled on your Roku device, and that your HDMI-CEC is enabled on your TV.
1. On your Roku Ultra, go to Settings/System/Control other devices/1-touch play (checkmark to enable)
2. On your LG TV, you will need to enable SimpLink. (this is LG's HDMI CEC). Depending on your TV model, you can usually find it under Settings on the LG remote, or by pressing the Smart Home button (one that looks like a house) on the LG remote, or look for a button on the LG remote named SimpLink. Make sure Simplink is Enabled.
3. On your Roku device, also try disabling the Power Savings feature if available on your model. This setting is under Settings/System/Power/Auto Power Saving/20-min inactivity (uncheck to disable).
When you have all the above configured, you should be able to press any button on the Roku remote (excluding the power button and volume/mute buttons), and the TV should power on, wake the Roku Ultra, switch to the appropriate HDMI, and you should find yourself on the Roku Home Screen. Keep in mind that the Power button and volume buttons on the Roku remote are TV controls only (they have no impact/effect on your Roku device). If you press the Power button, this will only power on/off the TV. It will not wake the Roku Ultra. Try pressing the Home button instead whenever you want to watch TV and have the above mentioned CEC features occur.
If you are still having issues, try using a different HDMI port and see if the issue may be with the HDMI port if you are doing all the above and having only intermittent success with the CEC 1-touch play feature.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941