I read through the documentation for installation and configuration of the plugin and also various posts in the community and found none with the same problem I am experiencing.
The URLs are the same as in both the WHMCS and plugin.
What is happening is that the SSL certificate is installed on main domain, which is what I am using, does not load when the link is activated WHMCS (cart).
I got to install wordpress plugin https, but this did not solve the problem because it carried the https but instead of showing the lock, showing a symbol of non-recognition of SSL.
Please, how do I solve this problem, since if I load the link directly by WHMCS, SSL perfectly shows the padlock and loading the plugin does not occur or https, only if I install the plugin https as I talked about earlier and shows in error.