Just to close the subject:
I haven't got any further. According to my research, I should be able to access the BIOS with ESC, F1 or something similar. In my case, however, nothing happens. The BIOS itself seems to be defective or no longer existing. By the way, the COMOS battery was completely empty. I guess standing around in a hot shipping container for months didn't do the device any good...

Thanks for your comments!