The inbound firewall is controlled a little bit differently.
The inbound firewall will deny any traffic that does not have a session initiated by a client behind the MX. This allows internal client machines to connect with any resources needed but does not let outside devices initiate connections with inside client machines.
https://documentation.meraki.com/MX/NAT_and_Port_Forwarding/Blocking_Inbound_Traffic_on_MX_Security_...
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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