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The below is from the internet about the cause of 502 error on nginx. 502 is an http error code about permissions which will block the gateway to the server. Jim should try pinging the server from the demilitarized nginx server to see if it can reach the firewalled server, I said this a few days ago. But here you go.

A 502 Bad Gateway error with Nginx indicates that the Nginx server, acting as a reverse proxy, received an invalid or unexpected response from the backend server it was trying to connect to.
 This issue typically arises when the backend server is down, unavailable, misconfigured, or unable to respond due to being overloaded.
 The error often manifests when Nginx cannot establish a connection to the upstream server, such as a Node.js application, which may be crashing, not listening on the expected port, or experiencing resource constraints like insufficient memory or CPU.

To troubleshoot, first verify the status of the backend server and ensure it is running and accessible.
 Check the Nginx error logs, which can provide specific details about the issue, such as "Connection refused" or "upstream sent too big header while reading response header from upstream".
 Review the Nginx configuration file to confirm the correct IP address, hostname, and port for the backend server are specified.
 If the backend is a Docker container or running in a Kubernetes pod, ensure it is on the same network and that resource limits (CPU and memory) are sufficient, as insufficient resources can cause the process to be terminated.
 Additionally, check for firewall or proxy settings that might be blocking communication between Nginx and the backend server.