The app will only work when both devices are connected to the same network, and there's no security enabled in the wireless settings of the router that block wireless devices from seeing each other.
If your Roku isn't connected to the network, it can't be controlled with the app. If the Roku in question is a Stick, the Roku remote is the only way to get it connected to a network. If it is connected, and the phone app is on the same network, then there's something in your wireless settings that is blocking the app from seeing the Roku.
When you look at the Roku screen on the TV, even though you can't control it, if you aren't connected to the network it should say so in the upper right side of the display. If it is any Roku other than a Stick, you can get an inexpensive universal IR remote to control it and get it connected to the network. Just make sure the IR remote states it supports Roku devices. Many, but not all, do.
Dan
Roku Community Streaming Expert
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