[CentOS] HTTPS certificates (off topic)

How do I get a valid certificate for a box that is behind a firewall and
does not have a DNS entry?

I was looking at letsencrypt.org but currently it looks like a valid DNS
entry is needed, of which I don't have.

There is nothing special about my setup, its just a box that is not
directly on the internet, no DNS entry but I need HTTPS to run correctly.

How do I generate a trusted certificate base on IP or something?

How can I do that? Thanks,

Jerry