This "app tracking protection" is just a DNS filter. You can achieve the same by setting a filtered DNS resolver like base.dns.mullvad.net
in the Private DNS options.
Auditor just verifies that your installation of GrapheneOS is real and unmodified, meaning it hasn't been tampered with by an attacker or corrupted in any other way.
I would recommend using a VPN. That's also why I prefer the DNS filter over something like app tracking protection, since it doesn't occupy your VPN slot. GrapheneOS only improves the actual Wi-Fi connection privacy (by randomizing your Wi-Fi MAC address), but it has nothing to do with the data transmission over the Wi-Fi network. That's what you need a VPN for. You can check out this comment about the Pros and Cons of VPNs, as well as the criteria for picking a good and trustworthy VPN provider: https://lemmy.dbzer0.com/comment/15631872 Here's some more advice about VPNs: https://www.privacyguides.org/en/vpn/