Solution to iPhone not connecting with 5GHz WiFi

To me that indicates that the Password used on the device that was unable to connect was wrong. Not sure if you entered this manually or one stored on that devices Passwords app or restored from the iCloud Keychain. The Show Network QR Code on a connected device includes the SSID and Password used without having to enter it manually or from a previously stored password.

Resetting Network Setting on the device that is unable to connect is a good way to clear out any previous WiFi passwords that may have been stored there.

Iphonewifisolution5Ghz wrote:

Many thanks for your thoughts but the issue was that the 5GHz network SSID was simply not appearing - so not even able to get to the point of entering a password, which in any event if had been able to do would have been entered correctly.

• Did you try to enter the SSID manually? In that case you enter the SSID and Password at the same time. Alternately, the Scan QR code option will use both the SSID and Password from a connected device.
• Is your router set to hide the SSID? They will still appear on a device that had previously connected.

I'm not sure this proves what you think it does. While many routers offer 4GHz and 5GHz bands, its typically up to the user to choose the band they want to connect to. And that means naming the bands separately, so the user can see the two bands and then choose which one they want to connect to. What you would want to do is to have named each band to something such as: "Home_Network-2.4GHz" and "Home_Network-5GHz." This is not done automatically.

Iphonewifisolution5Ghz wrote:

Precisely, that’s exactly the set-up: we have two bands (2.4GHz and 5GHz) with the SSID labelled accordingly to ensure that they are distinguishable.

Then Mac Jim ID's thoughts seem most likely the reason, which again, doesn't lead to an iPhone issue as you surmised.