How to get Let's Encrypt certificate files throught ISPConfig?

Hello,
I'm currently configuring a custom reverse proxy service and I need to automatically download the SSL files on ISPConfig to be used on the reverse proxy, which is in front of the ISPConfig (just like an Cloudflare service).
However, I've found that the fields "ssl_cert" and others are empty on the remote API when the website use Let's Encrypt. Also, I can't download the Let's Encrypt domain.com-le* files in the folder "ssl" in FTP, it says "permission denied".
Without any admin access level, would a client with basic access will be able at all to download his Let's Encrypt certificate? If yes, how?
At this point, I will probably do a cronjob in root to push the certificate to my reverse proxy service, but would like to have something else that doesn't require admin/root credentials.

 

What is in front should normally get the SSL certs on their own, at least that is how nginx proxy manager works.

Instead of mixing your intended custom reverse proxy, I'd prefer or suggest using ISPConfig full nginx or apache web server as the proxy manager to other ISPConfig server(s).