The availability of the 6 GHz band depends on your country/regulatory domain. If set to a region where it's not permitted, 6E options won't appear.
Also, ensure your CW9166I is running a firmware version that supports 6E. Although the device is capable by design, very early versions might not expose the 6 GHz settings in the Dashboard.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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