If you've been keeping your server up to date with patches, then (assuming you're using the homeserver.com domain) you have a free certificate for your server, which is specific to the "xxx.homeserver.com" vanity URL. When you access your server from inside your house, you'll see a certificate error, which is normal.
If you're using some other dynamic DNS provider, the opposite situation holds true: you will not see a certificate error while on your home network, but will from afar. You will need to work out how to get a certificate and install it yourself. In that case,
this how-to may be of some use to you. Warning: this is "at your own" risk; it's unsupported and can mess up your web server pretty badly if you do the wrong thing at the wrong time.
I'm not on the WHS team, I just post a lot. :)
