All TVs have a power button somewhere. It's often hidden very well. If it's not on the back, feel the area around the manufacturer's logo.
However, just turning it on won't do you any good. The app won't help - it can't connect to a Roku device that's not already connected to the same network. You need a physical remote. You can run to Walmart, Target, or Best Buy and pick up a Roku voice remote (which has to be paired to the TV) or get a universal remote that supports Roku TVs.
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