That is there for informational purposes only. You dont need to enter it there. You need to enter it in the mobile app instead of your ip address. Make sure to enter the port as well.
You obviously also need to setup a client for your
ddns service on a device on your network, pc, router etc, you only need it on one device. Most services have a pc client. You can also use BIT to update your ddns account (for the popular services)...