You can request them to unblock port 10000 (and port 20000 for usermin). If enough people ask for it, maybe they’ll listen.
And you can solve this by serving the domain directly without proxying it via Cloudflare. What I do with most of my servers is create a subdomain called say,
virtualmin.example.com and directly point it to the Virtualmin server and not via Cloudflare’s proxy (Make sure the cloud icon in your Cloudflare DNS settings is grey and not orange) for that subdomain.
You should now be able to access virtualmin via
virtualmin.example.com:10000 without any problem.
N.B. If the main domain
example.com is used solely for accessing Virtualmin, you can just toggle the proxy cloud for the main domain instead without having to go through the whole subdomain ordeal that I described above and use
example.com:10000 to access the dashboard.