The package add an rc.d script /etc/rc.d/isatapd to run isatapd as a system service. You need to modify it to set your router. Change the value of ISATAPD_ROUTER to the hostname of the router you want to use.
If you want to modify other options instead of using the default values, have a look at the project's homepage for further reference.
Start and enable isatapd systemd service.