Sorry for not seeing this post earlier. Were you able to get the API connecting from I3 to WHMCS? If not, be sure that your server's IP address is whitelisted in the WHMCS Configuration (Setup > General Settings > Security Tab). You may also need to add the IP to the whitelist found on that same page.
If that isn't working, please open a support ticket and we'll have a direct look at your configuration for you.