[SOLVED] No video signal after MB replacement (GPU & PSU checked) ?

[squeezyyy]

So after my whole story with enabling XMP and not being able to recover (I broke CMOS bat. pins), I bought a new exactly same motherboard (now more knowledge, so won't happen again, right?)
So now I get no Video Signal on my display. Even tho I know my GPU is perfectly fine (tested in service, and PSU worked perfectly before mb damage)

System specs:
MB: Gigabyte Aorus B550i PRO AX (mini-ITX)
CPU: Ryzen 5 5600X
RAM: Corsair Vengaence 2x8GB
PSU: CoolerMaster 850W SFX Gold
GPU: MSI Suprim X 3090 24GB (via riser)
Case: NR200P Max

What I tried:
1. Swapping ram slots (got only 2 slots and only 2 sticks)
2. Taking each one of them and running on slot 1 and then slot 2 (basically all combinations)
3. Flashing BIOS with Q-FLASH Plus to the lates BIOS version FDa
4. Resetting BIOS with reset pins on MB
5. Plugging GPU without riser

What I noticed:
1. PC is turning on in 2 different ways
a) Turns on and cycles in some loop with coolers speeding up and slowing down. In this mode it does not react to single tap on POWER OFF, only holding it.
b) Turns on and keeps static coolers speed. In this mode turns off immidiately after I press POWER OFF button.
I cant really tell correlation between how I get to any of those two, but they both happen from time to time.

What I think might affect:
1. Wrong installation of M2 SSD? (but still BIOS should've worked without, right?)
2. Missing some connection, cable I forgot to put into MB?
3. Dead / faulty RAM?

Can you guys suggest me my next step? Should I go and buy new RAM and try how it works out?
P.s. Dont have any friends or someone with PC parts to test on, so buying is unfortunately only option.

 

[squeezyyy]

Update: I took out my M2 SSD and put it back and everything is working now (maybe it was partially some other reason, but I changed only that)

Tho it's not very clear to me why would that affect video signal