The NUC is not working after BIOS changes.
Let’s try to get into the BIOS and reset settings to default, Intel has the following options:
1-While the start process keep tapping F2.
2-Power Button Menu. Press and hold the power button for 3 seconds then release it. Review details on page 55.
3-We can try to update the BIOS with the recovery option. It is necessary to remove the BIOS configuration jumper and plug a USB drive with the latest BIOS version then, turn the system on. The process is automatic.
Download the file fy0053.bio and keep it on the root of the USB drive
Ubuntu works as UEFI and Legacy. I recommend you keep both options enable.