I have tried all possible measures such as reboot, factory default reset (by reset button), reset by app, change the DDNS name, change deco, disable ipv6, DNS auto, DNS manual,  It just don't work.

I find the WAN address of DECO in the app then ping it. It works, even with ignore ping from WAN is selected?

Now I try using my DNS name xxxxx.tplinkdns.com, it doesn't work, it shows other IPs in a bracket with no reply.

Is it possible that ISP is blocking it? but how?

Any solution to this? thanks