Possible known failure modes of the WIU:
#1 Failed power input jack. The mini-USB jack can be ripped off the board cracking traces. It's possible and I have seen first hand where some power traces were cracked and ripped open while another remained allowing only the power light to appear as if the device had power. https://ogrforum.com/...t-wiu-power-jack-fix
#2 The module has become corrupted- often from power loss causing it to fail to boot. I've seen where even the reset that is supposed to revert to factory defaults does not work. At best, if you can reach the LUCI interface, then you can attempt to reflash the firmware. If you cannot get a network or get to LUCI, it may be bricked. One test option is try the wired ethernet jack if WIFI is not working.