Red led blinking 5 times, not turning on

I’ve seen some similar topics in here, but not exactly this product.

The red LED indicator on the front blinks 5 times and then turns off. There’s not apparent damage to the powerboard, it seems like the mainboard is defective, but how do I know for sure? I’ve seen somewhere that I should unhook the mainboard to check if the backlight turns on, is this right?