This has been quite a typical problem with the iRex DR1000, so maybe you can search this subforum and find a few hints you can adapt to the Onyx Boox, maybe this one:
https://www.mobileread.com/forums/sho...27&postcount=4
When I experienced this problem (with the iRex, not the Onyx) it was necessary to start the device while still connected with the power cable, and sometimes it helped me to press the reset button simultaneously.
Don't know if this helps but hopefully it gives you a direction what to try.