S7 is a stop train and not a metro. Stop train will arrive at the terminal.
There is more other train connection from Duesseldorf HBF to airport.
RE1 dirction to Hamm.
RE6 to Minden.
RE11 to somethere.
It is the first stop after Duesseldorf HBF and about 7 min by RE. But you have to change to a kind of shuttle bus or (H-Bahn) at train station (airport) towards terminal if you take RE. This will take you another 10 min.
For more info, you can look it up at www.bahn.de or www.vrr.de