Meraki support is correct and there is no other way to do what you want without a Radius server.
The only way I can see to do something similar is using Meraki's local authentication framework.
Let me ask you why you don't want to use a Radius server?
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.