Take a flight to CGN (Cologne/Bonn Airport)
Then take a taxi to Siegburg ICE-Station, about 15 mins from Apt. Let the taxi take you to platform 6, the open accessible paltform, where the train leaves..about 25 metres from taxi.
Buy ticket in train.
From there is a hourly (at least) train to FRA airport, which takes 36 minutes for the 164 kilometres.
Do not take a train/taxi to Cologne station, you will loose about one hour (taxi plus train) and some Euros for the longer distance.
Check on http://www.bahn.de for connection between Bonn/Siegburg and FRA.