In order to use a WIFIPLUG product your router has to be capable of emitting a wireless signal at a frequency of 2.4 GHz. Most, if not all, modern routers are capable of emitting this frequency but to check whether yours does or not, you can find this information on the product specifications for your model of router. Alternatively, you can contact your network provider or router manufacturer.
Every WIFIPLUG product is fitted with a 2.4 GHz wireless chip. This is what allows it to communicate with your home router and access the Internet. This chip only supports the 2.4 GHz frequency.
Why don't you use 5 GHz?
At the time our older products (WiFiPlug V.1, WiFiPlug2, WIFIPLUG MINI, WIFIPLUG POWER) were released the 5 GHz frequency wasn't as popular as it is today on consumer routers.
WIFIPLUG HOME on the other hand, is restricted to 2.4 GHz by the Apple® HomeKit™ specification requirements.