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.

[Buckeye]

I had issues with those NICs also, I don't even use them tbh.

What I had to do was as you said... force it to Auto and then the speed. These must match what you have set at the switch also. Still even after that, performance was poor.
Its easier to find a 10GbE NIC on eBay for $20 and use that.