With a default XAMPP installation -
If you are listening on 80, not using Named Virtual Hosts, and your router is properly forwarding, when you enter your routers IP from outside your local net, you should (unless you changed it) hit the New XAMMP Security Concept 'wall". That's why they put that 'trap' in there - 'anti-noob' insurance policy. Not pretty, but somewhat functional
IF your machine is multi-homed, or you have set up virtual hosts, those could be issues also
Did you actually try that DomainDossier tool Ms. Sharley suggested?
If so, what happened on the Service Scan???
I have other IP/Domain tools I use, but that's a nice one to add..