Disconnected Antennas seem to be a common problem, see:
You might be able to connect the antenna yourself. In order to do so, you can gently push the plugs on the antennacable into the sockets next to the silver chipset on top of the upper board (second photo, almost center of the image). AFAIK it is not relevant which antenna is on which socket.
I have no idea what that plastic-thingy is, though.