if you are able to run a public web server, then certificate issuance via certbot http challenge works pretty well. the web server can serve a really simple static page with little to nothing on it - but of course its another potential vector into your network.
if your public domain DNS makes use of a supported dns provider or you run your own publically accessible dns server, then dns certbot challenges are great and more flexible than http.
others may suggest neat work arounds for the http challenge issue, but if you have access to a supported dns service I would look at that option. certbot has helpers for quite a few public services as well as support for self hosted dns servers. I run my own public dns servers, so thats the option I chose and use certbot hooks, cron and bash scripts to rsync the updated carts to the propr hosts for the various services I run privately and publicly.