First make sure you have the right voltage going to the unit using a multi-meter.
If that is ok then you can suspect the network cable. Bypassing it, as suggested, is a good idea if you can. Or you can just re-terminatethe ends, using new RJ45 connectors, being really careful (eye strain) to make sure the right wires are in the right place on both ends.
My brand new 18HD was DOA when I got it but it still showed up, it just would not complete the initialization process. I did step one once, and step two multiple times before concluding that I had it right and the unit was wrong...
BTW... I have terminated some of my networking connections, radar included, with normal RJ45 connectors on the hub (GMS10) side. My GMS10 is in the cabin behind the helm station so it is pretty protected. I also goop up the connections with dielectric grease. On the outside you should stick with the garmin connectors. I still use the dielectric grease though.