I have several virtualmin servers that are very fast when accessing on home nework but are very slow or they don’t arrive at all (time outs) from my work.
I’m not able to contact either of those domains… whether via a web browser, or just pinging them… so if the server is online, there appears to be a network issue of some sort.
I’d first double-check that it still works from home, and verify that your server isn’t currently having a problem.
If your server isn’t experiencing a problem, you’d need to start looking at things between your server and the Internet… hubs, switches, routers, firewalls… perhaps even your ISP.