Mandatory DHCP
Mandatory DHCP requires client devices use DHCP for IP assignment. All 802.11ac Wave 2 capable MR access points running MR 26.0 firmware or later support this feature.
Enabled: Wireless clients associated to an AP (either new associations or clients that roamed from another AP) that have not requested a DHCP address are placed in a blocked state and are not able to send any traffic on LAN and WAN.
Disabled: The default setting for this feature. Wireless clients configured with static IPs are not required to request a DHCP address.
https://documentation.meraki.com/MR/Access_Control#Mandatory_DHCP
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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