Thanks a lot @piero and @aibd !
Strangely, it seems that making all the updates for Android apps (the system itself did not find any update) solved this initial issue. The smartphone was then recognised and I could proceed with the other steps.
I can now reach the step where I reboot the smartphone with “power+volume down+Bixby”.
But the install then stops right away and the final error message is
libusb: error [_get_usbfs_fd] libusb couldn't open USB device /dev/bus.usb/003/040: Operation not permitted
Failed to access device. libusb error: -1
On the S9 smartphone itself, there does not seem to be any error message although no mention of RMM State:
KG STATE : Checking
FRP LOCK: OFF
OEM LOCK: OFF
Secure download enabled
I tried the two USB ports on my Ubuntu labtop, another USB-C cable, and then even installed the easy-installer on another Ubuntu desktop. Same error message.
I did not yet try to put in a SIM card (but it seems to be more about accessing the S9).
One weird thing is that even though I accepted the computer as a trusted source for file transfer on the S9, every time the smartphone wakes from sleep it asks me if I am OK with transferring files from the computer. Besides this, I re-re-did all the steps listed in the easy installer.
I did check calling *#0808# that the MTP+ADB mode is selected.
Thanks again for the help & feedback!