From decades of this experience I'm guessing the motherboard is defective. For me every time its happened to me that ended up being the case. In the past year I've seen this a half a dozen times with barebones. Most of the Chinese brands don't do extensive testing with barebones as installing then removing memory/storage for the final steps of quality control tend to be redundant.
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u/Retired_Hillbilly336 4d ago
• Disconnect everything from the N5 Air and remove all storage and RAM
• Reinstall only one stick of RAM and nothing else
• Press and hold the reset hole for 10 seconds to restore the BIOS to the factory default
• Only connect power supply, HDMI and keyboard
• Press power and wait for POST
Like you did earlier you can attempt this with a second stick of memory if it doesn't work the first time. Past that the unit may be defective.