I bought a fake Samsung online at first it didn't seem fake when I opened it it was pretty fast. I checked it if was a fake just in case and it turns out it is.
Depending on your country and the country the device came from plays a big role with the above suggestion.
Hopefully you used PayPal to purchase the device. If so, open up a dispute ASAP and make a detailed description of the "fake" phone and explain how a legit one differs.
You can request a refund for the device. I wouldn't hold my breath on this one.