That message simply means that you don't have the Ethernet cable connected. Which I am sure you don't. Why is it looking for the Ethernet cable? Because it can't find any bootable disks.
If you don't have the BIOS password, you'll need to get it. You are unlikely to make any progress without it. I have seen eBay sellers offering a service to provide you with the master password for your unit, based on the alphanumeric Dell Service Tag code.
Once into the BIOS you can see if it recognizes the SSD
(that's what Mini 9 uses for a "hard disk"). It may be that the SSD
has failed. You might try creating a bootable USB flash drive (with Linux or something else on it) just to see if you can get that to boot.