I have discovered something very interesting. I was testing with the Samsung S21 Ultra also. It had the same result as the Windows machine: The 6 GHz SSID is never seen, even after scanning for many hours. I learned that someone else had connected an S21 successfully using Channel 37. I moved my AP to channel 37, and the S21 saw the SSID and connected. I then checked the Windows machine, it also saw the SSID and connected. I have been operating on a channel based on a specific frequency, which is a non-PSC channel.
The issue is that the S21 and the Windows driver will _only_ operate on the Preferred Scanning Channels. It is not just that it might take longer to scan - it will never see an AP on a non-PSC channel. So this means that for Windows and the S21, you can only use 15 of the 6 GHz band's channels (12 of which are available in the Asus - 37, 53, 69, 85, 101, 117, 133, 149, 165, 181, 197, 213) The Asus has a FAQ on PSC, but it doesn't say that client will simply not work on non-PSC channels.
Note that when using the Intel driver for Ubuntu Linux, the AX-210 will see the AP on any 6 GHz channel supported by the Asus.
The issue is that the S21 and the Windows driver will _only_ operate on the Preferred Scanning Channels. It is not just that it might take longer to scan - it will never see an AP on a non-PSC channel. So this means that for Windows and the S21, you can only use 15 of the 6 GHz band's channels (12 of which are available in the Asus - 37, 53, 69, 85, 101, 117, 133, 149, 165, 181, 197, 213) The Asus has a FAQ on PSC, but it doesn't say that client will simply not work on non-PSC channels.
Note that when using the Intel driver for Ubuntu Linux, the AX-210 will see the AP on any 6 GHz channel supported by the Asus.