Had the same thing happen to me.
The way I solved it was to turn the tablet on and let it settle for a minute on the DnX Fastboot screen. I then held the off button for 30 seconds (literally held it for that long), then leave it off for 5 mins, then switch it back on.
I know this doesn’t sound like it does anything but the 30 seconds holding the off button reset the fastboot mode so it won’t come up. If the update to the bios was successful (ignore the error you received, everyone gets that) your tablet will just start into the dual boot bios.