I had a similar issue with mine. My NUC is connected to my TV via a Yamaha receiver. I tried connecting the NUC directly to the TV and now I can see the UEFI screen when I press F2.
I think every time I need to change BIOS settings I'll need to connect it directly to the TV, and switch back to the receiver afterwards.
now I know the mistake. My TV don't support 1024X768, that's the reason. If I use a Monitor with HDMI-DVI adapter I can open the BIOS with F2. So I will use a Monitor if I must change thomething in the Bios.
Thanks for letting us know.