Sorry, but this is something on your network, the problem is not with WIFI.
I suggest that you review your network to validate who may be sending this splash page, especially because it is an external splash page, not the standard Meraki one.
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.