@Eja 

Hi, thank you very much for your time and patience.

May I know the MAC address of your main Deco?—you could send me a private message about the MAC address;

Just curious, since you mentioned that you could only control the Deco when it is connected to the Deco’s own Wi-Fi but the DDNS actually needs to communicate with the cloud server, so I think you might find the clue.

And when you on the mobile data, and try to open the APP, would there be any error message on the APP?

--If yes, can I have a picture of that?

Thank you very much and wait for your reply.