You're right. It takes several hours to accept your IP! It must be a manual process.
Also be sure to use IP on SERVER_ADDR and not REMOTE_ADDR.
What have confused me was the https://api.dynadot.com/api.html page not loading. One thing is to allow replies only from some IPs, other to deny complete server access based on IP.
No, The API is not dead that I know of. Dynadot would alert us via emails if this was true.
First error I find is that the Dynadot API needs to be configured properly in your settings to connect to the server. Look At: Account Area => Domains Tab => Advanced Settings => API -> Ip Address.
Make sure the right IP Address is there, double check with a site like: ipchicken.com
Next, it takes a bit of warming up. I find it common for 3-5 requests to go through unanswered (no server reply).
After these 10 - 15 seconds of warming up, The API kicks in right away saying: Success,, 1006644664 etc.. or not_available. etc..
same issue here, ive tried from 3 different locations, one in the US, the api host doesnt respond to http or https it does say beta but.. -> http://www.dynadot.com/domain/api.html