I use an external control panel for things (for example, using subdomains: sub1.domain.com and sub2.domain.com) that are on a completely different server. In order to make them secure, I have to ssh into my server and cat the ssl certificates every 90 days to keep up with the LetsEncrypt certificates. It would be nice to view and be able to copy the CRT, Key and bundle certificates from the ISPConfig 3 control panel without having to ssh into the server.
Look at the script by @ahrasis that copies certificate files. It uses incron to do the copy only when the files change. https://www.howtoforge.com/tutorial/securing-ispconfig-3-with-a-free-lets-encrypt-ssl-certificate/
That tutorial ia kinda outdated and though can be automatically set, it still requires ssh access to copy files accross servers. The solution you are looking is already suggested several times by @till which is NFS. There are a lot of tutorials on how to create NFS server and client which you can google out one of which is this: https://www.digitalocean.com/community/tutorials/how-to-set-up-an-nfs-mount-on-ubuntu-18-04
