I am sorry to hear you are having with your NUC but let me help you as much as possible.
I know that you just got the NUC and the components but is there any way to test a different RAM memory sticks? Also, can you confirm if using just one memory stick is working fine? If you use both the issue appears, I am correct?
I am sorry i do not have any other RAM to try
Using just one memory stick it works fine.
If i use both the issue appears.
I also have tried both memory sticks in single configuration it works fine.
Based on the steps done it is possible that the memory slots are not working properly when having both modules connected. Since you have the latest BIOS and you have done several troubleshooting steps, my best recommendation is to contact the place of purchase or your Intel® local support for them to help you with the replacement of the unit