Passive FTP from behind a router

[Xtreme2damax]

Hello folks, I have just followed the Perfect Server CentOS x86_64 with ISPConfig3 to set up a local machine as a server. I can connect via SSH to the server just fine but I can't ftp to the server, I have found out this is likely because the server and the client I am using is behind a NAT router but I am not sure how to resolve this.

I have disabled the built in iptables firewall and am using csf, all ports are forwarded in csf and my router.

Does anyone know how to fix this?