Custom Folder inside WordPress Root Domain

That should work out of the box: if a file or path exists on the file system, it should be served by your Apache webserver directly and not handled by WordPress at all.

Here’s an example: https://zepiwi.us.instawp.xyz/resources/downloads.html

I’d check your directory and file ownership/permissions.

That was a temporary demo site I set up just for you, which I must have since deleted. 6 days is a long time to respond to your own support ticket!

In any case, that was just an example. You just have to take my word that everything I mentioned is true and what you want to do is a very basic thing that should be possible.