Connection Refused when switching Cloudflare to DNS Only in an iptables C2 redirector setup. Why is traffic not reaching Nginx?

I'm working on a Red Team simulation and trying to set up a C2 (Sliver) redirection architecture using Cloudflare, Nginx (hosted on GCP), and iptables . I've run into a routing/connection wall and could use some fresh eyes. Target Architecture Flow: A victim machine (Windows) uses a PowerShell WebClient to send an HTTPS request to a target domain ( target.example.com ) with a specific HTTP header (<code