What the SecureW2 documentation implies is that it only supports certificates.
However, if they only work with radius servers, you can use them with a splash page.
See section V of the link I sent you.
You can also check this document https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Configuring_RADIUS_Au...
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.