Finally I set up Ubuntu on one of my old amd64 laptops and made some progress with fastboot on DB820c.
I am following the instructions Flash the Bootloader on the DB820c,
Flashing the bootloader want OK
When I boot into fastboot after a while the UART serial output stoped and can’t continue.
I tried it again holding the Vol (-) button a bit longer and I got the desired output.
Format: Log Type - Time(microsec) - Message - Optional Info
Log Type: B - Since Boot(Power On Reset), D - Delta, S - Statistic
S - QC_IMAGE_VERSION_STRING=BOOT.XF.1.0-00301
. . .
. . .
 fastboot: processing commands
I was able to successfully flash the bootloader and load the ALIP image.
I feel much more confident about the process.