From your description you have a domain registered, which points to the static IP of your router, which forwards traffic to the server?
However, if you are working from a 'local' machine on the same network as your Ubuntu box, they will both have 'internal' network addresses e.g. 192.168.0.X or similar, so any attempt to access the static IP will point you at the internet side of the router.
You need to set up a hosts file entry on the accessing machine to point to the 'internal' address of the Ubuntu box. In Linux/Unix /etc/hosts, in Windows - \Windowsdir\system32\drivers\etc\hosts.
Additionally, you Ubuntu box should have a properly confiured hosts file and hostname file, as per the Perfect Setup instructions.
/etc/hosts contents include
XXX.XXX.XXX.XXX myserver.mydomain.com myserver
where XXX etc is the server IP
On your accessing machine add
XXX.XXX.XXX.XXX myserver.mydomain.com to the hosts file.
Your ISPConfig setup should have that hostname file hostname as it's hostname, and the local 'internal' IP as it's IP.