As of Firmware 6.3, users can specify a custom Dynamic DNS server by entering its URL. To begin, navigate to Network > WAN and then select the connection you wish to modify.
Afterwards, scroll down until you reach the Dynamic DNS Settings menu. From there, click the drop-down next to the Service Provider item, and select Others…
From there, you will be able to enter the details for your Dynamic DNS server. Click the Save button and then the Apply changes button on the top right hand corner to implement your changes.