I wrote a little package for this because it was driving me nuts. Check it out here:
npm i @nick-bull/react-native-debug-address
# DEBUG_HOST=127.0.0.1:8081 npx react-native start --port 8081
# or, equivalently
DEBUG_PORT=8081 npx react-native start --port 8081
npx react-native run-android --port 8081