In the default React Native app scaffolding you have to edit AppDelegate.m to change where it loads the code if you want to test on your device. I use the following snippet to detect if it's being compiled for Debug or Production and for the Simulator or a device. For Production it uses a copy of the code included in the bundle, for Debug on the simualtor it loads from a server on localhost and for Debug on a device it loads from a server on a given IP address.
NOTE: You need to edit YOUR-IP-HERE and change it to the IP to load the code from when in Debug mode on a device. You could use a service like ngrok to make this work from anywhere.
NSURL *jsCodeLocation;
// Loading JavaScript code
#if DEBUG
// For Debug build load from development server. Start the server from the repository root:
//
// $ npm start
#if TARGET_IPHONE_SIMULATOR
// Run from locally running dev server
jsCodeLocation = [NSURL URLWithString:@"http://localhost:8081/index.ios.bundle"];
#else
// Run on device with code coming from dev server on PC (change the IP to your PCs IP)
jsCodeLocation = [NSURL URLWithString:@"http://YOUR-IP-HERE:8081/index.ios.bundle"];
#endif
#else
// For production load from pre-bundled file on disk. To re-generate the static bundle, run
//
// $ curl http://localhost:8081/index.ios.bundle -o main.jsbundle
jsCodeLocation = [[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];
#endif