Issue: Motherboard/PC won't supply power to externals (monitor, keyboard, mouse) and shuts off after flashing MB lights for >10 seconds. Occurred after switching to Secure Boot on BIOS.

Motherboard is: Gigabyte GA-Z270XP-SLI

The RAM, CPU, CMOS battery, Hard Drives, and everything else is a pre-built (iBUYPOWER) from a couple years ago. All of which are compatible with the current MB.

After setting the BIOS to Secure Boot enabled it has refused to boot/post. The MB lights alternate between CPU, DRAM, VGA, & BOOT before returning to CPU and turning itself off. Then it restarts the process a few seconds after. Repeatedly pressing what I presume to be the FLASH BIOS button on the chassis only switches the light between CPU and DRAM and prolongs it from shutting off but stopping for even 5 seconds makes it shut off again. Could this be a faulty PSU or PSU cord perhaps? Is the motherboard shot and a replacement motherboard is needed? What would the solution(s) be in this instance? You can suggest the previous attempted solutions in case I erred. Regarding flashing the BIOS, I have a microSD 16GB and a 4GB SDHC both formatted in FAT8/16/32, if I need to use those instead.

Attempted solutions:

Replacing & Reseating the CMOS battery (old battery was put back in). Reseating the GPU & only using one (in the 1st slot). Reapplying new thermal paste to the CPU cooler. Jumping the CLEAR CMOS with a screwdriver for 20 seconds at least 4 times. Reconnecting the GPU and motherboard cords. Flashing BIOS on the red USB port using an exFAT 64GB flash drive reformatted to be FAT32, the largest file in the zip for the latest BIOS file was renamed to be GIGABYTE.bin and was placed in the root of the USB alone.

Manual for this MB: https://download1.gigabyte.com/Files/Manual/mb_manual_ga-z270xp-sli_e.pdf?v=624dac4248a165d8176043560d5eb1e1

BIOS/Driver info for this MB: https://www.gigabyte.com/Motherboard/GA-Z270XP-SLI-rev-10/support#support-dl-bios

Q Flash Method explained by Gigabyte: https://www.gigabyte.com/FileUpload/global/WebPage/20/images/utiltiy_qflash_uefi.pdf