I mean that a bunch of the front end code assumes that you’re using https. I mean that the standard install assumes that your site can get a certificate from let’s encrypt.
Here’s one. For this to work, you’ll need to set up apache with a valid certificate and then have it reverse proxy Discourse.
Set up Discourse on a server with existing Apache sites
It’s not a supported setup. If being behind a firewall/NAT is a requirement, then having someone who knows how to set up an internal reverse proxy with a valid certificate and can follow one of the guides like the one linked above is the cost of that requirement.
That’s a nicer way of saying what I said.