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 they need, but does not let outside devices initiate connections with inside client machines. The exception to this is if a Port Forward or 1:1 NAT is created. More information on Port Forwarding and 1:1 NAT can be found here.
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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