@Lucas_T as explained by @KarstenI you need a /29 network assigned, If you wnat configure public IP on WAN interfaces.
The only way to use the same public IP is using a NATed IP. Take a look at my example, I only have a single public IP, but my wan interface is configured with a private IP, i.e. it's behind a NAT.
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I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.