All Z-Wave devices that plug directly into an AC wall power outlet, such as smart switches and dimmers (for example the GoControl Plug-in Dimmer Module or Aeotec Dimmer 2), act as "repeaters."  

You can also buy a repeater as a stand-alone accessory device (for example, the Aeotech Range Extender).   

If your Z-Wave device does not appear to be communicating with the controller, try plugging in a repeater approximately halfway between the Z-Wave device and the gateway.

For additional support: