You need to have statistics collection enabled on your S3 gateway. The S3 gateway will save the statistics as regular storage objects. On each S3 storage node, create a file /var/lib/ostor/local/gw.conf with the following contents:
# Enable usage statistics collection. S3_GW_COLLECT_STAT=1
Restart the S3 storage service to apply the configuration changes. Run the following command on all S3 storage nodes:
# systemctl restart ostor-agentd.service
Now you can login to WHMCS. Additional links and S3 management options will be shown in the Client Profile section.