WPA3 Transition Mode
WPA3 SAE has a transition mode (sometimes called mixed mode) created to allow WPA2 clients to co-exist on the same SSID used for WPA3. Although WPA3 needs to have Management Frame Protection (MFP/802.11w) set to Required, the Dashboard can also be set to Enabled, so that the STA which are not compliant with either WPA3 or MFP can still connect seamlessly.
802.11w can be set to Required, however WPA2 clients which do not support MFP will not be able to associate.
If you continue to have problems, I suggest creating a new SSID.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.