May I know which page do you want to open?

The domain name I created using tp-link ddns is

www.xxx.tplinkdns.com

where "xxx" stands for the actual name I used. I thought that if I could open this page I would
be able to access my deco network from a browser. But what I get is

This site can’t be reached

If this is not how it works, can someone explains how it works?

what device do you connect the main deco to, a modem? did you open some ports for a certain server in the network that you want to access it with the domain? and also check the IP address that your domain name bind to (cmd-nslookup xxx.tplinkdns.com). if it's a private IP, you need to open port for the deco on your modem or something.

the deco is connected to my modem. the rest is beyond my expertise. I wish tp-link would provide a more detailed use of this feature.

Usually it works with port forwarding. You need to open a port for your internal server, then you can use the DDNS to access the server remotely. http://www.tp-link.com/us/faq-1797.html
But be care for about the WAN IP address under Settings>Advanced>IPv4. If it's a private IP address, then you need to open the port for the deco unit on your modem.