How to reach NH Dusseldorf City Centre Hotel from Dusseldorf Airport?
You first take the train to Düsseldorf Hbf (main station). It is a 5 to 10 minutes ride (depending on the number of stops) and there are some 6 to 8 trains/hour.
From there, you can either walk about 10 minutes to your hotel or take the 709 or 719 tram to the "Berliner Allee" stop and walk from there.
Taking a taxi from the airport to the hotel is of course also an option and will cost you about 20€.
I don't know why Google Maps suggests to take the 707 tram all the way. That sounds like one of the least attractive options.