Network Controller light can lit with just power supply connected. Platform should boot right after plugging DC cable and print messages on COM1. If it is not doping so, probably firmware is corrupted. If You have a SPI flash recovery dongle, You could just connect it to SPI header and boot platform. If even this solution will not help, I would suspect hardware defect on electrical level. I assume You have nothing connected to apu except serial cable and DC cable.