No BIOS access... because of a monitor?

I recently built a new pc and all is well (except for my dislike of Win 10).
During the building and setting up, I used what I will call 'Monitor A'.

A few days ago I decided to connect my older monitor (Monitor B) to
see if I preferred it or not. I used the same HDMI cable for both monitors.
The resolution is the same for both. The biggest difference is that Monitor B
has a much lower refresh rate than Monitor A.

Now the tricky part. At first, I didn't really notice that the pc (with Monitor B)
booted with a black screen before the desktop loaded. When I booted the pc
usinng a (Macrium) bootable USB stick, all I get is the black screen. I don' get
the boot menu with F8. There is no boot screen, no blinking cursor, no nada.
I restarted the pc, tapped 'Del' to get into the Bios and all I got was the same black
screen. I had to hit the reset button to get back into Windows.

So I disconnected Monitor B and reconnected Monitor A. It booted with the
familiar Asus boot logo ... then into Windows. I had no issue getting into the
Bios or boot options menu. I've switched monitors 3 or 4 times and this behaviour
is consistent.

Why on earth would I have this weird behaviour with Monitor B? Why would
the Bios care which monitor is connected?? Its a head scratcher.

TIA

I'm not sure what you mean by 'set to use HDMI'. AFAIK, HDMI is auto-
detected.

It works -- just no boot screen or Bios access display.

Ok checked the menu, all I could find was digital input set to on. Nothing specific to HDMI. Never been in that menu before
as I never had the need.

- - - Updated - - -

I also meant to say that I tried accessing UEFI from within Windows, but was still
left with a black screen. Reset to get back into Winidows.

Probably it is more related to the settings of the videocard and less to the Bios settings.

If you press F10/Del (or whatever key your board uses) when the screen is black, does it go in?

@PiKo
What settings should I be looking at?

@MrBasher
Its F2 or Del on my board -- neither has any effect. I even tried Enter for giggles.
Just found out that Ctrl/Alt/Del does restart the pc, so Im learnin' somethin'. LOL

Thanks for the help!