On your Meraki Dashboard, if you hit the question mark in the top right corner, you'll see a link to Firewall Info.
This will show you all the relevant IP Addesses and ports on upstream firewalls, neccessary for proper Merak Cloud connectivity.
These openings are different network to network/organization to organization, and may be subject to change, depending on which products you have in the Dashbaord. E.g. If you have Meraki Cameras, you'll see entries in the list for IPs and Ports required by the MV Cameras.
You should lean towards what your own Dashboard informs you of neccessary IPs and Port openings.
Tldr; make sure udp/7351, udp/9350-9351 and tcp/443 are open outbound from the Meraki MX, as well as ICMP...
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