EX7500

Hi i have just purchased a EX7500 WI Fi extender as i wanted to extend my WI FI range into the garden in my small bungalow.

However when i plug it in and the 5 lights turn blue my Wi FI signal gets less it goes from around 30 meg down to 15 meg download speeds when it is switched on. switch it off and the speed goes back up to the 30 meg again.

What could be wrong? why would this happen?