This setup has been tested successfully on GL.iNet GL-MT300N-V2
device with OpenWRT v22.
By default OpenWRT firewall does not allow web or SSH access on WAN IP. As a captive portal blocks all traffic on LAN until user is authenticated so when we initially enable captive portal on OpenWRT we lose access to the router from LAN side. To manage our OpenWRT we should therefore open access from WAN side.
Access your OpenWRT router via Luci interface. Go to Network => Firewall => Traffic Rules and add a new rule for opening port 80 (HTTP):