The remote's power button seems to reverse the power state of the TV. But then why is there a TV Off shortcut at the bottom of the on-screen channel grid, with a whole page of support devoted to it?
You can use a Roku Simple Remote (no Power or Volume buttons) to navigate to it. Or the Roku app. Or a third party universal remote that may or may not be programmed to the TV as well.
DBDukesRoku Community Streaming ExpertNote: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."