When you setup your domain registrar you get the following error:

CURL Error: 7 - couldn't connect to host

Things I have done:
- Configured API settings at reseller club (with support checking IP was correct too)
- Configured WHMCS Register settings (ID and password checked!)
- Contacted my server host (Curl is functioning fine!)

It is most likely then that you are giving the registrar the wrong ip address. For instance when you are running a website on a shared host such as justhost.com and you have your own dedicated IP address it is not this address that communicates with your registrar, your dedicated IP only really workd for http traffic.

Solution

• Download this CURL IP Checker and see what IP address is actually being used.
• change the IP in the registrars whitelist or appropriate section to match the required IP address
• original thread here