How can I get into fastboot mode?

At first, I tried to run fastboot mode, but I failed to do it.
So, I got the file of and unzip it. Then, I succeeded to flash the bootloader into the dragonboard-820c. After doing that, I cannot connect the board between host PC via USB. Besides, I can’t get into fastboot mode, too.
So, I tried to recover my board using SD card recovery image(dragonboard-820c-sdcard-rescue-38.img), it didn’t work though. :frowning:
I don’t know how to fix it. Plz help me to resolve this problem. And, I would like to go into fastboot mode and install Ubuntu 16.04 in dragonboard-820c.

Do you have serial console ?

Did you configure switch to boot on SDCARD (OFF-ON-ON-OFF)?

Yes, I do. Also, I did configure switch (OFF-ON-ON-OFF) when booting on SD card. Unfortunately, it doesn’t work at all. How can I recover my board and install Ubuntu?