Don't know if this will help but spearson gave me these instructions when I lost the F2 bios prompt when I misconfigured the BIOS on my D54250WYKH.
"Power off your machine and then power it on holding the power button down for 3 seconds. This should pop up the recovery menu, which allows you to get control back. Note that holding the button down for too long (4 seconds) will result in hard power off, so you may have to try this a time or two to get the hang of it..."
Just got a NUC5i7WYK at work and it has some "teething pains" when using Win7-64 Pro. We were not able to use the latest BIOS since it wouldn't boot (flashing dash in upper left corner of display).
Sorry for these dis-joint responses... When we had the BIOS issues with the new BIOS we tried reinstalling Win7 and there was an attempt to repair the Windows installation from USB boot drive. This repair damaged the Win7 install files on the USB. I had to re-burn the Win7 image on my USB using the Microsoft Windows USB/DVD Download tool Windows USB/DVD Download Tool - Microsoft Store.
I was able to get the recovery screen, but it seems my problem persists.
I've tried three different USB keyboards and none seem to communicate successfully with the NUC. When I press F2 or any other key, it simply ignores all input from the keyboard. It appears to be quite functional otherwise... but will not make it far enough to install windows without a keyboard. (Of course, we know that one USB port is working... since it was reading my optical boot disk through that port, but somehow keyboards are being rejected.)
If your usb keyboards are wireless, apparently there is a problem with using them via a usb dongle directly inserted into a nuc usb port. Don't know if this is true of wired usb keyboards across the board, but my nuc 5i5ryh wouldn't work with a wired microsoft usb keyboard or wireless logitech usb keyboard. If yours are wireless, I've read using a usb extension of at least a foot to your dongle works. Hope that helps.