Captive portal without changing the router's firmware

I could find a way to create a captive portal using OpenWrt, but I'm thinking of a method to do it without changing the firmware, is that possible? What is it about?

Does the router's parental controls really block all Internet traffic?

I am using the Asus 4G-AC53U router. When I turn on "Parental Controls" to block a client's Internet access, does the router actually block ALL types of Internet traffic, including system service (such as checking for operating system updates, checking licenses, etc.)? .)? Or just block the user's normal access, for example, Web browsing, video streaming, etc.?