The 802.11w amendment applies only to a set of robust management frames that are protected by the Protected Management Frames (PMF) service. These include Disassociation, Deauthentication, and Robust Action frames.
Some legacy devices that do not support 802.11w may not be able to connect to an SSID even if in mixed mode. This may be due to the device improperly handling the advertised information contained within the beacons.
802.11r uses Fast Basic Service Set Transition (FT) to allow encryption keys to be stored on all of the APs in a network. This way, a client doesn't need to perform the complete authentication process to a backend server every time it roams to a new AP within the network. Thus avoiding a significant amount of latency that would have previously delayed network connectivity.
802.11r is intended for use on SSIDs that use enterprise authentication methods.
Fast Transition (FT) 802.11r roaming is not supported between Meraki MR55/MR45 and any other MR Access Point (AP) running version 25.x or lower. If you have a mixed deployment with MR55/MR45 and any other model of Meraki APs and 802.11r is either set as enabled or adaptive on any of the SSIDs configuration ensure all your APs are running version 26.4 or higher.
In both cases I suggest leaving it as is (disabled)
About DHCP I actually wanted to know if it is configured as NAT mode or Bridge mode?
About WLAN, I suggest creating a new one (not changing the existing one) only in 5Ghz, to test this 802.11ax client.
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.