The captive portal works at layer 2 so if you have problem at layer 3, ip address - routing etc, the captive portal is likely the only thing you will see. Check that the pfSense DHCP server is handing out the correct IP address etc. I confess that I don't use the captive portal anyway currently so I'm perhaps not the right person to answer this. Anyone else?
I'm not sure what you mean by 'ath0-WAN'. You should have one WAN, alc0, and one LAN, ath0.
Steve