I have recently built a gaming rig from all brand new parts (besides the GPU which is used, but perfectly functional).

Specs:
Mobo: Gigabyte Z390 Aorus Master
CPU: i9-9900k
RAM: 32GB G.Skill Trident Z Neo 3600Mhz CL16 (2x16GB) f4-3600c16d-32gtznc
GPU: EVGA RXT 2080Ti XC Ultra
PSU: Seasonic Prime Platinum 850W
Boot drive: Samsung 970 Evo Plus 1TB + second exact same drive

After a lot of troubleshooting, asking on forums etc the problem persisted: i am unable to get any signal from the video card when its in any of the PCIE slots, doesnt matter if its through HDMI or DP. I am unable to even get to BIOS, when the card is in there is completely no display at all, and also with the card plugged in (and BIOS settings at "Auto"), i dont get video via onboard, meaning the mobo detects the GPU and automatically disables the IGPU. During that time, the "VGA" LED on my mobo is lit up, meaning it sees that there is SOME problem, although the debug display shows "A0 (IDE Initialization is started)" code (might also be AD - which is "Issue Ready To Boot event for OS Boot"), and the same is displayed when I'm fine in windows via onboard or another GPU.

Everything runs just fine if i take the 2080Ti out of the PC and run it just from the IGPU, zero problems, instant boot, everything detected, even managed to OC the 9900k to stable 5.1Ghz, out of boredom.. The 2080Ti is fine, i used it for the past two months with my old rig (Z77 mobo and i7-2600k) and it was running perfectly okay, i also tested it in two different PCs (also using my PSU in those PCs, to eliminate the PSU as the problem) and it works just fine. I flashed card's bios to the newest one in my friends PC, didnt help. I also tested a 2070 and a 1060 in my new rig, boots fine with no issues, got instant video via DP. I tested the power cable, PSU, i tried if the card is seated in the PCIE slot correctly, nothing helped. I uptaded mobo's bios, no change.
If i manually force the PC to use onboard via BIOS, i am able to get video with the GPU plugged in , but the card itself doesnt show up in BIOS, it doesnt show up in device manager, and if i try to install GeForce experience, it says i have no Nvidia GPU plugged it. All of that is quite weird because card's RGB is lit up, the fans are spinning and the backplate is getting warm when i touch it over the die.
I also reassembled the thing outside of the case, i reseated the CPU to make sure it was fine, checked the inside of all PCIE slots, they look fine inside. I tried three different PSUs and different cables, to no effect.

Coming to the conclusion - i did a whole ton of troubleshooting, tried every single common fix that people suggested, nothing worked. The problem is still the same with zero change - everything runs fine, until i plug my 2080Ti in my system, then i get no video via any output..
I totally ran out of ideas, everything is fine by itself. The 2080Ti is fine and working in another rig. Mobo's PCIE slot is fine too, as it works perfectly fine with other GPUs plugged in. Also the monitor and cables are fine, im certain.

I hope someone can help me finally fix this without RMAing anything as i'm already very frustrated by the time waste on all of this.

Do you have another hard drive of some type you can install a fresh Windows 10 on to rule out any OS configuration problems? 

I have an Aorus board myself.  My issue ended up being the USB ports I was using for my keyboard and mouse when I booted up the first few times.  It was weird.  I kept moving them around and eventually everything booted up properly.

Try unplugging the power from the card.. the pcie power connections from both ends and reinsert them. Possibly they are not fully seated or a wire pushed out the backside of the connector.

PCIE power connectors are fine, im certain about that.

I dont think i screwed the cooler too tight, its a Be Quiet Dark Rock Pro 4 which has special mounting brackets, would be hard to overscrew in those. I'm not using any cables, the system is sitting on an open-bench now, as barebones as it can get. I will try doing the UEFI thing now, maybe it will fix something?

thank you for the update