We have done some work recently to make this scenario more manageable. I’ll explain the details below, but before we go there, I’d recommend updating your firmware to the latest version of Model01-Firmware. That should work out of the box for many BIOSes, and for those where it does not, you should be able to hold Left Fn + Esc + Shift (assuming the factory layout) to put the keyboard into a mode compatible with older BIOSes.
The gist of the issue is that there are two modes a keyboard can report what keys are pressed: boot mode and report mode. The first is what pretty much everything supports out of the box, but it has a limitation of only being able to accurately report 6 keys pressed at a time (thus it is often called 6KRO, short for 6-key roll over). The second, report mode is what people refer to when using the term NKRO (N-key rollover).
The USB spec says that keyboards should start in report mode, and the host must tell them to switch to boot when it can’t speak report mode. This… doesn’t always happen. The spec also says that despite report being the default, the host should nevertheless, tell the keyboard to switch to report mode. Needless to say, this doesn’t always happen, either. There are a number of other complicating factors that make it next to impossible to both support the keyboard working in BIOSes, and NKRO under supporting operating systems. We have a bit of logic to make it as painless as possible, and make it work most of the time. For the times it does not, the Left Fn + Shift + Esc combo (all on the left half) will toggle between boot and report mode, thus enabling you to put the keyboard into boot mode before entering the BIOS.
Mind you, there may be a chance that the BIOS does not recognise a keyboard if it is “plugged in” after entering the BIOS. In this case Left Fn + Shift + ESC will not help, and we do not have a solution for this case yet. I think we still can figure something out, but we’d need to determine first if we’re dealing with this situation.
So, I’d suggest you first try an updated firmware, and see if it works out of the box. If it does not, try the Left Fn + Shift + ESC combo. If that still does not work, then try entering the BIOS, plug in a keyboard that normally works in it, and see if the newly plugged in keyboard is recognised. If you do not have enough keyboards (you’ll need one to enter the BIOS, and another to plug in later), then plugging the working keyboard out and back in might still exhibit the same problem… And that’d give us enough hints to proceed further with the issue.