Tried it Legacy (DOS mode explicitely does not exist/not mentioned) - doesn't work.The BIOS needs USB Legacy support (DOS Mode) enabled to boot from a USB device.
Same for me... I have tried all kind of low-power boards in the past, starting with C3 CPU's from VIA. Theses were a real nightmare.I have only found a few boards with UEFI that will handle legacy USB well. Most being Intel or SuperMicro. I don't think I've ever had success with any off-brand.