To clarify, when an SSID is configured with Sign-On Splash page enabled, the two settings that must be configured for unauthenticated clients to have full network access and not be allow listed are:

Controller disconnection behavior: This setting determines how the clients are treated when the Meraki cloud controller is unreachable. The options areRestrictedorUnrestricted. The former option blocks all traffic from unauthenticated clients until the controller is reachable again.The latter option allows unauthenticated clients to access the network without signing on until the controller is reachable again1.

Captive Portal strength: This setting determines how often the clients are redirected to the splash page for authentication. The options areBlock all access until sign-on is completeorAllow non-HTTP traffic prior to sign-on. The latter option allows unauthenticated clients to access other protocols such as DNS, DHCP, ICMP, etc., but blocks HTTP and HTTPS traffic until they sign on.This option is recommended for compatibility with devices that do not support web-based authentication1.