This is our mega guide on letsencrypt: Let's Encrypt, Nginx & Reverse Proxy Starter Guide - 2019 Edition
Most things will apply but some things are a little different due to addon gui settings vs commandline.
A couple of things to keep in mind are, the container name on libreelec will be docker.linuxserver.letsencrypt so to see the logs, you do "docker logs docker.linuxserver.letsencrypt" instead of "docker logs letsencrypt"