How to root au Qua Tab PX (LGT31) sold in Japan

[thumbshifter]

au Qua Tab PX (LGT31) sold for Japanese market is a variant of LG G Pad X 8.0.
Does anyone know how to root this device?
In particular, does anyone know how to go into fastboot mode?
"adb reboot fastmode" command and hard reset screen do not enable the fastboot.
"fastboot devices" command or other commands do not work.
Are there any special operations to enable fastboot?

 

[thumbshifter]

When executing,
adb reboot recovery
the device reboots to a screen with a robot and "No command".
Neither of fastboot command or adb command works from there.
Can you give me a clue?

 

[salmanateeq]

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