Question New PSU and GPU, system not booting !

[Jeff_120]

Hi
I received today my brand new Asus Rog Strix 850w PSU as well as the Zotac 4080.
I started by replacing the PSU and its cables, then the 4080.
Pressed on power button and...nothing! Just the motherboard rgb light is working.
I unplugged and repluged the 24pin cable 3 times to be sure it's well plugged, same for the cpu plug , and the 4080 I plugged it twice and repluged it to be sure it's well seated and the 3 power cables are connected to the new nvidia gpu adapter.
Nothing !
What could I miss?
Could the PSU is DOA?

 

[--SID--]

Did you flip the switch on the back of the PSU?

Reset CMOS of the motherboard.

 

[Jeff_120]

Did you flip the switch on the back of the PSU?

Reset CMOS of the motherboard.

Took it to a PC shop, the technician jump start the PSU by using the 24pins trick and it booted up from bios settings .
He said it should work now .
Brought it back home and...it didn't boot up
Now this PC shop is closed and I have no clue what to do!
Looks like the bios didn't like the new PSU?
Should I jump start it myself and change something in the bios? Perhaps I need to reset the gpu settings in the bios to accomodate the new 4080?

 

[Vic 40]

I started by replacing the PSU and its cables, then the 4080.

Replaced all cables? Also are the cables well seated on the psu side as well?

 

[Jeff_120]

Replaced all cables? Also are the cables well seated on the psu side as well?

I replaced the gpu, cpu and motherboard cables, the Sata and molex are the same seasonic numbers b55 and b57 that's why I didn't change them
I just jump start the PSU and the case fans worked , obviously not the rest as the 24pin and geforce power were not plugged . So I can rule out the PSU
I tried to jump start the motherboard but no joy
There is something forbidding the system to start , either the cmos or a bad plug and I can't find out which one

 

[Vic 40]

Maybe check motherboard see all cables are rightly connected like fan cables and front i/o.