One option if you don't want to request more IPs from your ISP is to see if it's possible for their router to be configured as a private network and for them to do an outbound NAT to the internet.
This way you can configure the WAN IP of each MX with a private IP instead of a public one.
I'm not sure if that's clear?
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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