closest solution to your problem is here:
set your local IP for testserver.linuxhome.net that will be fixed (static)
in ISPConfig set in:
Default Ns1: and Default Ns2: to dns servers of you ISP provider.
hope this will solve the problem.
you can always later change ip in /etc/apache2/vhosts/Vhosts_ispconfig.conf
192.168.0.100:80 to *:80
if you are using GoDaddy: