After some searching and testing weeks, I have the right OTG cable found that works in both directions.
A SAWAKE 4 Port Hub is doing its battery service and ensures that the data flow in both directions. This is a milestone on the way to my remote station. The OTG hub cable combo running at the moment still in test mode. Connected are an external hard drive and the ELAD FDM S2. The hard drive is powered by a power hub with the SAVAGE 4 Port Hub.
Audioengine D1 is apparently not supported by the USB output.
However, an unsolved problem always means that more arise.