Mounting the AP3915e to a junction/gang box requires a WS-MBI-WALL04 bracket, which must be purchased separately.
Note
When you line up the holes, the locking tab on the bracket must be pointing up and the junction/gang box must be fully covered by the bracket. The bracket must be square to the rest of the room walls and the two holes that are being used must be on the opposite sides of large center hole on the bracket.