Your phone/tablet must be connected to the same network as your TV. In addition, if your TV uses a wired connection and your phone/tablet uses Wi-Fi some routers have a setting to isolate wired and wireless devices and it's possible that it could be configured to isolate all wireless devices, but I don't think that's likely. If that's not the case you can try using your TV's IP address displayed under Settings->System->About and entering that into the app manually.
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