It's not the only option, but in this case he must have to configure the walled garden setting the IP of his external page.
Manually specify an IP range or domain names users can access prior to sign-on. Meraki's splash page is automatically included in your walled garden. If you host your own splash page you must enter its information here.
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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