You can't access using the WAN IP from within a LAN but you can access using the IP of the computer that XAMPP is installed on for example http://192.168.x.x
or what ever IP has been assigned to the XAMPP host computer in the LAN.
The local loop back address 127.0.0.1 will only access XAMPP on the computer it is installed on.
From home you will be accessing via the internet using your ISP created (WAN) IP.
In version 1.7.3 of XAMPP the security settings for this behavior are in \xampp\apache\conf\extra\httpd-xampp.conf file under the heading New XAMPP security concept, <LocationMatch>
and Allow from
Best wishes and I hope this helps with understanding how it works.