Raspberry Captiveportal, avoid android's splash page (iptables rules?)

I'm trying to add to my Raspberry captive portal some offline (locally hosted) functionalities (a website), but it doesn't work on the minimal-browser that pops up when a "blocked" connection is sensed by android. I searched online for a solution and i've already tried to whitelist the urls checked by the system (**google**/generate_204) but the popup keeps opening. I don't know if my whitelisting rules were bad or what, but i can't find a way to stop that browser to open unless i completely unblock the connection (which i don't want to). Any tips on how to make it work?