Your ceritifcate files should now be in this directory: /etc/letsencrypt/live/myhostname.no-ip.org/
I also assume your Plex server is port-forwarded to be accessible via port 32400: http://myhostname.no-ip.org:32400
Before we begin, we need to generate a PKCS #12 (.pfx) file from the Let's Encrypt certificate files. It's all the Let's Encrypt files archived, and bundled into one file.