@GaryTan did you get to solve this issue ? I am having the same problem, i opened all the ports in the router, assigned a static IP, allowed DMZ, portforwarding.....etc. and i can not use it. I changed internet provider (because i moved) and i am having this issue since then. 

 Dear @Solla-topee, 

The probleme shooting you refer were the first steps i've tried. After that i've tried everything else since i am still having connection issues. If you could please check my new post i've made (since i still cant resolve the problem) to check what the diagnose log says : https://community.tp-link.com/en/smart-home/forum/topic/710672 
 

I've reviewed the log an the most repeated errors are : 
1- 4 HTTP - HTTP Client Send Buffer is full 

2- 4 HTTP - Close Session Called 

3-4 P2P -  P2P free session  (Session_ID)
4- 2 CloudIoT MQTT conn err 1006 - MQTT Event 4 err, stas 0 
5- 2  HTTP P2p dbg - UDT state (6) 

Thanks for your attention