FTP Server using VPN

I run an FTP server from one of the PCs on my LAN which I use to connect from outside (WAN) to download private files.

After setting up OpenVPN client on my RT-N66U flashed with the latest Merlin firmware, I am now unable to connect to the FTP server from outside my LAN.

Would it be possible to someone to provide a SIMPLE guide as to how this can be achieved please?