Reboot (20sec holding power button, or fastboot reboot command) until screen flashes (or you get USB disconnect on computer if screen is unresponsive).
Then press and hold volume UP + DOWN for ~15 seconds? If screen is responsive you will be with boox logo on screen, so at first sight no change from fastboot. But device will be detected by EDL utility.
If on linux, there is easy way to see events on USB bus with dmesg -W. There is usblog utility at
Renate's website for windows which does similar job.