I use this project (https://github.com/FiloSottile/captive-browser) which works most of the time.

Most captive portals work by answering the DNS requests with the captive portal ip. This works only if the correct dns servers are configured and a lot of security features like dnssec, DOH, ... are disabled.

More info from the project author: https://words.filippo.io/captive-browser/