How to use Rekordbox USB Stick with new Rekordbox Android?

I am a DJ. My record collection is saved on a 256GB external USB drive (Rekordbox format), which can be connected to USB or USB-C. 

For 3 hours now I am trying to use the new Rekordbox 4.0 Android mobile and feed it with my Rekordbox DJ Stick, where my track collection is saved. It does not work.  

I start Rekordbox Android on my smartphone and plug in the USB stick in the USB-C connector. The USB Stick is well connected and can be read as I can see when I click on files and look for the files on the USB stick. I can use the content of the USB stick with several programs (VLC player etc) But in the new mobile Rekordbox Software 4.0 Android I cannot open the playlists/tracks from the USB stick. The choice of sources is Rekordbox/Soundcloud/Tidal. Where do I find the source 'external USB flash drive' to play tracks from my external flash drive?