Go to Help > Firewall Info. It will list the IP addresses that you'll need to allow through your firewall. Most of them are outbound rules, so unless your firewall is restricting outbound access, you can ignore those. After you've specified your RADIUS server's information and have a setup configured for RADIUS authentication, you should see an entry for inbound traffic associated with RADIUS. As you'll see, you only need to permit UDP port 1812 (you can do 1813 as well) for the Meraki IP addresses listed in that entry. See attached for an example.