you should use Manual Setup in Step 1, and manually put the device into force recovery mode.
Or you can also flash with flashing scripts provided in our BSP:
It is possible to ping 192.168.55.1 on my host PC.
But I don’t know how to check is my device booted into ubuntu.
And I have another question, should the device be connected separately to the mouse, keyboard, monitor, and LAN line when installing through SDK Manager?
That sounds weird. If you are able to access 192.168.55.1 (which should also mean you can SSH into it), then SDK Manager should be doing well.
Anyway, if you still fail with SDK Manager, just try this one:
I think it’s not needed, but connecting the device to a monitor can help you confirm it is booting into Ubuntu properly.