How do I forward client IP instead of proxy IP in Nginx Reverse Proxy?

Overview

I have a setup that looks like this:

• Nginx Reverse Proxy with a Floating IP
• Reverse Proxy is configured to forward requests from users to a 3rd party SaaS application, BigCommerce which then handles the response to the user.

I’m unable to set Trusted IPs/Proxies in the BigCommerce application environment, therefore the client IP is always showing as the last connecting proxy instead of the client IP. Due to how ordering in BigCommerce is tracked, all orders are being marked as ‘potential fraud’ because of an IP location mismatch between the IP being logged and the customer making the order.

My Setup

I’ve been asked what headers I’m sending via the Reverse proxy and it looks like this:

proxy_pass_header  Set-Cookie;

proxy_set_header   Host               $host;
proxy_set_header   X-Real-IP          $remote_addr;
proxy_set_header   X-Forwarded-Proto  $scheme;
proxy_set_header   X-Forwarded-For    $proxy_add_x_forwarded_for;

From what I can see and have been shown from the BigCommerce, the X-Forwarded-For headers are being sent with the correct IPs in the correct order (client_ip, proxy_ip), but X-Real-IP shows as the proxy_ip instead of the client_ip.

Their suggestions have been to override the X-Real-IP header from the Reverse Proxy and I can’t seem to be able to do so. From questions I’ve asked in other online communities, I’ve received suggestions on overriding the $remote_addr with proxy_bind, but that requires running Nginx as a superuser and messing with IP tables.

My Question

Is it possible that the droplet configuration (Possible NAT issue?) is what’s causing the IP to show as the proxy_ip instead of the client_ip or is what I have configured missing something crucial in my setup to ensure the reverse proxy doesn’t overwrite the X-Real-IP with its own public IP address.

Any help or suggestions are greatly appreciated. I’ve also looked into proxy_protocol, but I don’t know if that fits my situation and if that requires BigCommerce to support proxy_protocol in order to correctly identify the client_ip.