@Joe, Yes that I have gathered, from your reply and from the attempt at getting a *.example.com from LE. - Never again!
However, I’m still confused about sub-servers (doh! my numbscull)
the site needs to be found at 991.example.com (example.com is up and running with a different site)
I tried setting up a new Virtualmin “top level” site as 991.example.com (This of course fails to obtain a LE cert) so I tried giving it a name of 991_example.com because Virtualmin does not care
but that will also fail because the domain doesn’t exist (might as well be abc.abc)
As “sub-server” worked for 999.example.com I assume it should for 991.example.com and so I’m back to not understanding what the difference is.