P5530: can't boot from external DVD or USB flash drive

I want to revert to a Windows 10 System Image created about a year ago using the Windows 7 legacy backup utility.

I have the bootable disc (created at the same time as the Windows image, option "Create a system Repair Disk") both in a USB external DVD drive and an a 8 GB USB flash disk (stick); none of them show up after F12, ony UEFI boot is possible, no external device is listed.

In the BIOS (1.17.1), Secure boot is disabled, Enable Legacy Option ROMs is selected and Boot sequence is set to: Legacy External Drives
- USB storage device
- CD/DVD/CD-RW Drive

What am I missing ???

Help, this is pretty urgent