The remotes below are both versions that will work with the DC400 - the ABCD remote must display the blue light when any of the remote buttons are pressed.
Troubleshooting your remote
Do any other remotes work? if yes and it is only the one remote not working, try changing the battery. If the light comes on on the remote and it still doesn't work you can try re tuning to the device. If it still doesn't work then you will need a new remote.