X570 Phantom Gaming X won't turn on

What was the reason for getting the NEW PSU?

Did your old one break?
If not, try using ur old one again, if it works your new ones appears to be DOA.
If your old one broke it might have killed something else inside your PC.

I installed new psu Corsair RMX 850w gold today. PC won't turn on whether I pressed the case power button nor the mobo power button. The psu fan does not start turning at all. All I see is the mobo power and reset button red LED lights flash briefly when I turn the PSU power button on and off, so power is still going from the PSU to the mobo. I already made sure that:

24pin is connected securely to motherboard
8pin cpu power is connected to top left of motherboard.
Ryzen 3700x and wraith prism cooler are slotted in securely.
2x8gb ram is slotted tightly to the 2nd and 4th slot from left.
chasis power
System Panel Header are all plugged in correctly in the bottom right of motherboard

I omitted GPU for the sake of PSU and motherboard diagnosis. I do not have any peripheral plugged in during this diagnosis. I don't know if the inability to boot up is my mobo or the brand new psu's problem.