Asus Prime X299-Deluxe II w/ faulty Aquantia 5 GbE NIC

[Growly]

For the second time, the brand-new Prime X299-Deluxe II I've received doesn't have a working 5 GbE NIC. No connection is detected when I plug a cable in. I'm using CAT 6 cables known to work at 1000 gbps with the switch at the other end, since I can plug them into the onboard Intel NIC and they work fine.

I've tried updating BIOS, drivers, CMOS reset and Linux (Ubuntu 18.10).

I asked Asus support for help and they told me to RMA the board, so I did. Now the replacement has the same problem.

This is extremely poor effort by Asus. This board was $500.

Has anyone else had this problem? Found a resolution? Is it something stupid like no auto-crossover or something undocumented like the fact that it doesn't actually work at less than 5 GbE?

UPDATE: Plugging the NIC into the other NIC finally gets me a link up. Something about auto-negotiation with the switch is busted. There are no MDI/X settings I can tweak in the driver and I'm not about to go and buy a crossover cable. I tried forcing a link speed to avoid negotiation problems at the switch, but it didn't help. This seems to be broken by design. Maybe if I ever get a 5 GbE NIC it'll be worth something.