Use case: roscore runs on some robot R. Joystick is on some remote client C

Make sure you've exported the master, and the IP of C.

On C: rosrun joy_node joy, then rostopic echo joy. When you move the joystick, you should see some output.

On R: rostopic echo joy. When you move the joystick, you should see some output. If not, then maybe the IP of the client C is not known to the master.