@Walterjnr 

Hi, I happen to see that you have set up DDNS from the cloud server so I wonder both WAN IP and DDNS failed to work, or just DDNS?

Can I have some pictures about how you access the NAS, like the access path and the error screenshots when failing to access?

Thank you very much.

@TP-Link 

Thanks for the reply. DDNS resolves just fine to my IP address but they both fail to connect to the NAS from outside of the network. Internally the NAS connects fine and is running as it should. I just can't connect to any service that is running on it (externally). I have a few ddns services running and I can ping to the modem using them but as soon as I specify a port it times out. 

@Walterjnr 

Thanks for the update.

In this way, Could you please send me some pictures of your port forwarding configurations, and some screenshots when you failed to access NAS externally?

PS: I have sent you my email address via private message.

Thank you.

@TP-Link Thanks again. I have emailed some screenshots of my configuration with the Deco app. In terms of connecting to my NAS remotely, this is mostly done via browser and the only error is a timeout or "can't connect" message so these screenshots won't shed much light. 

@Walterjnr 

Thank you very much for the pictures.

I see that the IP address under IPV4 is not a public IP address which means out of the Deco X20-DSL, there is another firewall which might block the remote access to NAS.

Have you checked with the internet service provider to see whether they could open the same port for Deco’s  IPV4 IP as well?

@TP-Link  Thanks for all of your help. My ISP implemented a CGNAT and I have now opted out. Everything works so far.