How does the 9800 turn off the wifi6 to keep only RF below wifi5?

[Translator]

The customer's equipment is relatively old,  and it is found that the SSID of the wifi6 signal cannot be searched. Can only the radio frequency of wifi5 be reserved, for a certain SSID on the 9800?

[marce1000]

 - Use   GUI:
           Wireless > 802.11 a/n/ac/ax > High Throughput (802.11 n/ac/ax) > 11 ax mode [uncheck]
                                 Repeat same for 802.11 b/g/n/ax

 M,

[Scott Fella]

IF the customers equipment is old, what exactly do you mean by this?  The 9800 is not old, but are you talking about the access points that are joined to the 9800?  6GHz only works when your SSID is configured for 6GHZ and you meet the requirements and that your ap has a 6GHz radio.  Maybe I'm not truly understanding your question.

[Rich R]

WiFi 5 and WiFi 6 use the same RF frequencies and channels - the changes are in 802.11 standards.

More likely that your clients are affected by something like:
https://www.intel.co.uk/content/www/uk/en/support/articles/000054799/wireless.html
which simply requires driver updates to resolve.
What exactly is the equipment having the problem?

[JPavonM]

If you mean to disable all WiFi6 related features and IEs on the packets becuase you are corcerned about really old devices from been imapcted by them, then do what @marce1000  propose and disable dot11ax network, or you can disable all dot11ax features under the specific WLAN profile:

wlan <YourProfile> <id> <YourSsid>
 no dot11ax