We found out the the SD card which stuck in the Pinephone was not readable anymore, and we do not know what was on the SD card.
We erased it, and installed JumpDrive on it according to the instructions of TechHut
Now, we where able to plug in our PinePhone to our PC, and the PinePhone is recognized as an external drive.
This lets us remember to the good old times, when this was possible with Android phones, which we could plug-in via USB to a PC, and the external SD card of the phone was recognized by the PC and could be used as an external drive from the PC.