Check the model number on the spare receiver. If its an H24, HR24, H25 or an HR20-100, then you need to take off the white box and ethernet cable
The H24, HR24 and H25 both have the white box built in. The HR20-100 needs a special hook up. You have to add a splitter to the line going to the receiver, then run 1 cable with a bandstop filter to SAT IN 1, and a 2nd cable with the white Box to SAT-IN 2.
Also, if its an SD receiver D11 or higher, you'll need to take the white box off and replace it with a bandstop filter (D11's cant access the internet or whole home).
Long story short, let us know what model it is and we can give you more specific instructions.