Copy the Sucuri Cloud Proxy IP address Range and sucuri.net domain name htaccess code below into the Plugin Firewall > Plugin Firewall Additional Whitelist Tools > Whitelist by Hostname (domain name) and IP Address text box > click the Save Hostname and IP Address Rules button > click the Plugin Firewall Activate button.
sucuri.net, 192.88.134.0/23, 185.93.228.0/22, 66.248.200.0/22, 2a02:fe80::/29, 208.109.0.0/22