Net result being that two different GPU's do not work in your current PC. Correct?
However, the 1080 ti (provided that I have correctly followed everything

) apparently does not work at all.
I think you are narrowing down the potential culprit.
If no GPU's will work in the current PC then the current computer/motherboard is suspect.
If a given GPU does not work in any PCs then that GPU is suspect.
Will the 1080ti work in some other known working computer other than your own systems?
That is important to know.