Thanks for your response, Biosham.
Yes, I have had disabled secure boot in the Security / Secure Boot menu of the BIOS on my Chuwi Hi12, and also used F7 key for the boot menu. However, the F7 key boot menu did not offer to select the USB DVD drive.
Basically that is not a surprise to me, since I have not been able to set the system boot order in the BIOS Boot / FIXED BOOT ORDER Priorities page to my USB DVD drive.
Offered devices are USB Lan, USB Key, USB Hard Disk, Hard Disk:Android-IA. But not USB DVD Drive. Although my USB DVD Drive is known to the BIOS. It is recognized in the BIOS / Advanced tab / USB Configuration as Mass Storage Devices: Optiarc DVD RW AD-7580S FX20.
The question remains: Is there anybody who is experienced with booting into a Live DVD on a Chusi Hi12?