Düsseldorf airport has its own long distance train station, served by regional trains and a few long distance trains. This train station is connected by the so called skytrain with the long distance train station. There is in addition the Station "Düsseldforf Airport Terminal" underneath the terminal. From there you can take the S 11 to Düsseldorf main station.

Depending on where and how you would go to Munich, take the S 11 to main station as more long distance trains stop at main station than at the long distance station at the airport.