So let me make sure I understand your setup:
You have a SSID set up with a splash page with Meraki Cloud Authentication set up with a username/password set for specific users?
Are you trying to track traffic by user? Is there a reason you're using username/password instead of a shared secret WPA2 password? Do you have a WPA2 password set as well?
Can you share some screenshots (with passwords concealed, of course) of the SSID settings?