This screenshot shows a 4G connection. How are you trying to resolve(use Pi-hole) via 4g/LTE ?
I don't think there's a way to bypass the Carrier (Cell provider) DNS via iOS (unless jailbroken or via VPN).
You can use this guide to bypass the carrier's DNS setting when using cell data connection:
This will allow use of carrier's full speed connection and Pi-hole resolution for DNS (I've been using this for months now).