No that's not what I write. Here is the normal situation:
PRTG on server myserver.local port 843 (443 is already in use) - this is fine internally, but for external this could be mydomain.com port 843 as I don't own the myserver.local domain.
A device, for example mobile phone internally would be set up the connection to https://myserver.local:843 and would work fine. However the moment they left the internal connection e.g. left the building, unless they use a VPN then that URL would cease to work. They would need to switch to https://mydomain.com:843 - not very convenient, and all the emails would be wrong as well.
So to prevent this and also make life easier as who wants to open and expose unnecessary ports externally, I have set up a web proxy on the firewall (HAProxy to be exact). It forwards both internally and externally https://prtg.mydomain.com (port 443) to https://myserver.local:843 - the "prtg" subdomain tells the proxy which server to connect to internally.
This works perfectly for both internal and external connections as the URL does not change, but as I stated in my original post it does not get reflected correctly in the emails sent out by PRTG (and the link it displays during an upgrade).
If there's a better way to do this I'm happy to hear your ideas.