Don,
The web relay device should not have anything to do with being configured for DDNS. It only needs a private IP address on your remote network.
You can get a sense of this simply with your computer that you are using to read this post. In your web browser, Google "what is my IP address". It will come back with your public IP address such as 185.37.136.66. But now go to your local network settings on your computer and you will find that your PC's network address is something like 172.20.2.130. This second address is called a non-routable or private address because you cannot ever directly reach it from an external Internet connection. The address ranges that are reserved for private networks are 10.0.0.0 - 10.255.255.255, 172.16.0.0 - 172.31.255.255 and 192.168.0.0 - 192.168.255.255.
It is the job of your router to convert this private address to the public IP address (such as 185.37.136.66) whenever your computer needs to access something on the Internet. The router also keeps track of traffic coming back for your PC and converts the public IP address back to your PC's private IP address so that your PC gets the message or packet. This all happens transparently and without any need to configure your local router to perform this task. This function is commonly called NAT (Network Address Translation).
Now with regard to your web relay device, it will normally not initiate connections to the external Internet. Rather, you will want to remotely initiate a session with the web relay via the Internet. Because your router at your remote shack will see this message or packet, it needs to know that this type of request should be directed to the private address of your web relay device. This is usually done by manually configuring a port mapping table (commonly called a port forwarding table) in your router to tell the router that if any port 80 packets come in, forward them to the web relay device via its private address. Port 80 is the common port for a device that has a built-in web server but check the documentation as it may be on a different port.
I hope you can see from this description, that the DDNS function is completely separate from the web relay device and so the device needs no knowledge of your public IP address, of your URL that maps to that public IP address, or even the fact that you are using a DDNS. You only need to have one DDNS client running somewhere on your remote network - either within the router itself or on a connected computer that is always on. This little client simply monitors the public IP address of your router (the functional equivalent of Googling "what is my IP address") and when it sees that it has changed, the DDNS client sends a message to your DDNS provider (such as no-ip or DynDNS) that your public IP address has changed. Then when you use the URL for your remote site in your web browser (such as
http://remoteradio.ddns.net), this automatically gets changed behind the scenes to your latest public IP address (such as 185.37.136.66) to properly route the message to your remote shack router. The router looks up port 80 in its mapping table and sends the message onto your web relay device via its private IP address (such as 172.20.2.130).
- Glenn W9IQ