@Isan_28 

Hi, the modem in front of the Archer C6 router is a modem and router combined, right? If so, the WAN IP address on the Archer C6 would be a private IP address.

The DDNS domain name will be bound to the real public IP address, thus it would be different from the C6 Router WAN IP address, to confirm whether the DDNS is working or not, please use nslookup command to check which IP address the DDNS domain name resolves, then search what is my IP online to find your public IP address, if the two addresses are the same, which means the DDNS is actually working fine, please let us know the results.

For more troubleshooting about the DDNS function, please follow this guide: 

Troubleshooting Guide of DDNS function on TP-Link Wi-Fi Router, LTE Gateway Router or Deco Router