I used this guide: https://github.com/xcp-ng/xcp/wiki/Create-a-local-ISO-repository
On the xen DOM0, create a place to store all ISOs, max 18gigs of space will be available.
Then:
Created:
It shows up in xcp-ng center:
Then, use git bash scp or FileZilla to sftp the files over to the dom0 /opt/mylocalisos directory, so that xen can have them.