Have you tried to do a jumper-based BIOS Recovery? Instructions are here: Intel NUC BIOS Recovery Update Instructions.
When you power on without DRAM installed, does the Power Button LED flash to indicate that memory not present? If not, then the board has failed.
Do you have a mDP-to-HDMI adapter to check whether the alternate graphics output is working?
I already tried the jumper-based BIOS Recovery but probably could not make it run truly as keyboard seems not to be activated or recognized and no dis play was available.
When powered on without Dram, the Power Button LED doe not flash.
I didn't know anything about mDP-to-HDMI adapter and don't have one. I don't if it will worth trying.
That's a mini-DisplayPort to HDMI Adapter. There are a number available cheaply through Amazon: Amazon.com: CableCreation 4K Mini Displayport (Thunderbolt Port Compatible) to HDMI AV HDTV Male to Female Adapter for M…
Hope this helps,