Load Balancing through another brand load balancer

When you say “the connection is breaking”, are you referring to the entire application? I’m not clear on the issue.

From your explanation, the first thing that jumps out is the nginx box may not be configured to pass the x-forwarded-for header to the backend dB servers. Even if they decide to put nginx in front of the webservers instead of the F5, they may encounter similar issues. Every device in the stack has to be aware and able to extract x-forwarded-for headers if app logic is built on the real client IP address.