That is exactly what the plugin does. No need to make a symlink and do anything from the command line. It creates the cert with certbot and then imports that cert into the ssl cert section of the OMV web interface. If you want to use the cert for ftp, it should be in the dropdown in the ssl tab. If you want to use it for the omv web interface, look in the web administration tab. No idea about mumble but maybe you could point it at the /etc/letsencryp/live cert.
Ah hmm , It looks like that for some reason when the letscencrpt certs gets renewed I still see the old expired cert under Certificate SSL.