This is due to a problem with your server, nothing to do with the networking.
Try checking your error logs including apache2/nginx error.log and mysql error.log. If your error logs say such as cannot allocate memory, then you should consider upgrading your droplet to one with more RAM.

Rebooting your server should fix the issue but you need to dig deeper if it keeps happening.

This happened to one of my local sites a few months back. There’s a really easy to follow guide here at https://mythemeshop.com/blog/fix-the-error-establishing-database-connection-in-wordpress/ that has a video as well on how to solve it