If the app cannot find your Roku or the buttons have no effect, check the following.
- The phone and the Roku are on the same Wi-Fi network (watch out for « guest » networks or split 2.4 / 5 GHz bands).
- The Roku is turned on: it cannot be controlled while off.
- On the Roku, allow network control: Settings → System → Advanced system settings → Control by mobile apps → set « Network access » to « Default » or « Permissive » (not « Disabled »).
- Run the scan again, or enter the Roku's IP address manually.
- As a last resort, restart the Roku and your internet router.