NextDNS Blocks Porn Site Fine but Not in it's Own App

A DNS service provider, like NextDNS, converts easy-to-remember website names (like websitename.com) into their corresponding IP addresses (like 111.222.3.444).

When you use a dedicated app that you've downloaded on your device, it connects directly to the IP address of its servers without needing to go through the DNS lookup process. This is because the app already knows the specific IP address of the server it needs to connect to. 

If you want to block specific IP addresses from accessing your network or device, you'll need to use a firewall service . This could be firewall software installed on your device, a dedicated hardware firewall for your network, or the firewall utility built into your modem or router.

You will need a firewall to block that app from communicating to its server. 

If someone can download and install a porn app, it means they would be able to install a VPN or proxy software potentially evading any blocking you want to set up. Unmanaged devices can't ever be easily controlled from outside.