You may not have got it then, try many times before you decide it's not working. I know that may be hard to time out since nothing is on screen, but once recovery starts the screen should come up as it would be booting from the other BIOS.
Did you ever try the onboard graphics yet, or another graphics card? I'd try the onboard for sure, but then also another card if possible, just in case this is a graphical only issue as you should be able to see the screen with the CMOS / Checksum error.
Shorting the motherboard as I mentioned will not initially require graphics, first boot will be main BIOS and a failure to boot, then it will shut down the board and reboot from the backup BIOS only. Then from there, yes you would want to be able to see the screen so you can know if it's allowing you in the BIOS, or auto recovering the board, ect.
Before I link you to any of that though, please test the onboard graphics and another graphics card as well if you can find one an old PCIE or PCI graphics card would be fine, just try to find something from this decade and it should be usable
But try with the on board graphics for sure, as in do that now if you have not already
Also! Please verify that monitor is still working properly, do that on another system, and while trying the onboard test use a different monitor connected to this machine if you can too.