When using an external keyboard on Android, there is a bug that input is not working if the on-screen keyboard is visible.
The workaround is to set the on-screen keyboard to not be visible when using an external keyboard. It may be different for each model, but you can roughly follow the procedure below.
- Go to "Settings"
- Select "Language and Input"
- Select "Default Samsung keyboard" under "Keyboards and input methods" section
- Turn off "Use on-screen keyboard"
- push "Setup input methods" button