Big update.
Tried Fedora 42 Workstation. 6 GHz works fine.
Same thing with Linux Mint 22.1.
Appears there is something wrong with the recent Ubuntu releases. They break 6 GHz WiFi on the AX210 for whatever reason.
I asked about this over on the Intel support forums, but did not get a useful answer. They don’t know their own product. An Intel person tried to tell me that 6 GHz functionality is only available with the AX210 under Windows, which of course is ridiculous.
I’ll be using either Fedora or Mint on the machine. I haven’t gotten anywhere trying to figure out what’s wrong with Ubuntu 24.04/25.04. I’m tired of banging my head against the wall.
Once you get 6 GHz working, it’s pretty spectacular. I can use the full 160 MHz channel width, so transfer rates are about double what they were at 5 GHz with an 80 MHz channel. Neighborhood of 2100 Mbps.