Hmm, Ok. So it sounds like the APs are in fact behind pfSense. Traffic from clients on wifi goes through pfSense and out on one of the new high speed WAN connections you have and not through the Cisco firewall.
If doesn't really matter what path the authentication takes the captive portal doesn't manage that it manages traffic from the clients to the internet. It should be on whatever internal interface the APs are connected to. You will have to add pass rules to allow the APs to reach the cloud controller though.
It also sounds like you don't want authentication on the APs at all. Clients have to login at the captive portal anyway.
It does look like the Meraki APs support radius accounting so you could probably do limited connection time per user there directly but if you need to set bandwidth limits per user or use total data limits I think you would need to use a captive portal.
This is not something that we often see though.
Steve