tias
Alternatively you can configure your router/modem in bridged mode and use sme in server & gateway mode & let sme do all the work, and have an easier time managing your sme too !
The firewall in sme will be as good if not better than your hardware router. The sme kernel gets regularly updated and your hardware device stays at an old release (on chipset), it probably uses a Linux kernel too.
You have full control of services, ports & firewall iptables rules etc by using the combination of server manager settings, db commands & command line/custom template tweaks.
It's simple to enable ftp in the server manager, takes about 1 minute.
No need to open & forward ports as this happens automatically when you enable ftp ie services get enabled & the firewall rules get adjusted accordingly.