Following 3 ways to publish the FTP on the Internet for your reference:
1. Public Fixed IP (most easy, not recommend)
Just assign a public fixed IP on the NAS WAN/LAN1 port.
2. DMZ (easy, not recommend)
Enable Router DMZ function and fill in NAS WAN/LAN1 private IP in the Router DMZ column.
Router WAN IP is 126.96.36.199
NAS WAN/LAN1 IP is 192.168.1.100
Enable Router DMZ function and fill in 192.168.1.100 for DMZ, thus the Router will forward all connection of 188.8.131.52 from internet to NAS WAN/LAN1 IP 192.168.1.100.
3.Virtual Server (port forward) (a little difficult, recommend)
For method 2 and 3, we also have a quick guide for reference, http://esupport.thecus.com/files/Thecus_Internet_Quick_Guide_20100701_v1.0.pdf
Assign port 20 and 21 forward to NAS private IP. If user enable secure FTP (Explicit), user have to configure passive port range and forward on the router. (Default value is 30000~32000).