Firstly, make sure if the following settings are correct.
Do you set a fixed LAN IP for your camera?
Do you set DDNS service settings and DDNS status shows succeed?
Do you do port forwarding in your router?
Do you active your DDNS domain name?
Secondly, if you finished all the settings above successfully, please check if there is more than one router or modem in your LAN or not. If you connect two or more routers in the LAN, you need do two times port forwarding.
About how to do two times port forwarding, please see the guide of How to do two times port forwarding.