WHMCS can be a little overzealous when it comes to security; it will occasionally ban IP addresses from logging in and requires that an IP address already be on a whitelist to access the API. This guide will provide a quick tutorial to whitelist specific IPs in WHMCS. If you’re looking to whitelist an IP in Bitninja or on an individual server, not a single WHMCS install, then follow this guide instead (you may have to end up doing both).
Whitelisting an IP address
After you get the IP address in question, log in to WHMCS and under the Setup tab hit General Settings.
Next, find and hit the Security tab.
Scroll down and find the Whitelisted IPs section. Here, click Add IP , and in the pop up box fill out the IP  and a reason for whitelisting it  (I recommend using the ticket number the request to whitelist the IP came from) . Once everything is filled out, hit the Add IP button on the popup box  and the IP will be exempt from being banned for invalid login attempts.
It will be the same process for whitelisting an IP address to use the WHMCS API. Just provide the IP and a note to why the IP is whitelisted. I recommend if at all possible to use the hostname of the machine being whitelisted, or at least the ticket number the request to whitelist the IP came from.
Removing a banned IP address
Removing an IP is a little more complicated. You’ll have to do some database stuff. You can either run this command (replacing banned IP address with the actual banned IP address) on the server the WHMCS install is on.
echo “use $(echo “show databases like ‘%_whmc%’;” | mysql | tail -n 1); delete from tblbannedips where ip = ‘banned IP address’;” | mysql
Or you can just open up PHPMyAdmin on the cPanel account WHMCS is installed in, and find the tblbannedips table in the WHMCS database (which is usually username_whmc1).
Then next to the IP address in question, hit delete.