Apple TV Failed Activation

I have an AppleTV 3rd generation ... ever since I restored it I cannot use AIRPLAY from either my iPad or iPhone. It previously worked fine but I decided to restore it because when airplaying it was playing up!

Now when I try to airplay from my iOS device it says looking for AppleTV. It cannot find my AppleTV.

When going to AppleTV SETTINGS / ABOUT / its say Activation FAILED.

I have tried everything from restoring via iTunes, unplugging it, resetting ... you name it I have tried it but cannot pass the Activation Failed. Hence I cannot airplay. Everything else works fine on the AppleTV - i.e. apps etc

Does anybody know how I can correct this and why this is happening?

Any help is much appreciated.