From Captive portal descrpton:
A Captive Portal consists in a gateway that is the default router for the subnet to protect. Such gateway blocks IP packets destined towards the outside and captures the http and https requests on TCP ports 80 and 443 redirecting them to a web server (called Authentication Server) that show to the user an authentication page. If the user insert the right credentials, the Authentication Server communicates to the gateway that the host of the user is authorized and the gateway forwards the packets outside of the protect network.
That’s why it is impossble to use Captve portal and transparent proxy on the same router, IMHO.