You can just use the dynamic hostname, that way whenever the WAN fails it'll be updated to whichever WAN is active. Depending on which DNS server the actual client uses, this is pretty speedy.
You can see the DDNS name we assigned to your MX by going to Security & SD-WAN > Client VPN. If you don't see this option, go to Security & SD-WAN > Appliance Status and click "Configure Dynamic DNS" on the left hand side
Kind regards,
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Connor Loughlin
Network Support Engineer
.:|:.:|:. Cisco Meraki EMEAR 🇬🇧
For reference, many questions can be easily answered by searching our online documentation: http://documentation.meraki.com