If you need an application to help test connections, I'd recommend you try Packet Sender. This application is free, cross-platform, and allows you to send and receive UDP and TCP data both through a GUI and command-line interface.
This makes it perfect when testing new configurations, especially if you need either targets for your load balancer, or to test the connections through NGINX. Here's what the GUI application looks like:
As shown in the screenshot, we sent a quick ABC123 packet to the NGINX server and received I received: ABC123 back from our upstream application (which is a simple echo app).