Hello,
If you just want to start mosquitto from a router reboot. You can use the command @Reboot /etc/init.d/mosquitto -p port number to use. You would not need another script to do this.
If you want to modify the mosquitto default configuration, you can access the file through the path /etc/mosquitto/mosquitto.conf ,and enable this preference with the @Reboot /etc/init.d/mosquitto -c /etc/mosquitto/mosquitto.conf command.