SSH to router and execute command as below.
opkg install luci-app-ddns
Login to router after installing luci-app-ddns package. Dynamic DNS is added to the Services menu as shown in figure below.
You must register a DDNS service such as dyndns.org, I suggest to use no-IP.com since it is free of charge. After register your desired DDNS service, enter all the required information in the Dynamic DNS page as similar to figure above.
Save the settings and reboot the router, upon restart the router, your router public IP address will update to the DDNS service site (it may take a few minutes). You can use ping command from DOS prompt to verify your IP address.
Picture below is an example of using ping command and dyndns.org as the DDNS service. Please note that ipconfig /flushdns can be omitted, it make sure clear the cache from your Ms Windows.
The result (IP address) from ping command should be same as your router public IP address (use this link to check you public IP address).