I'm using the WEB- and FTP services from Internet
Can you share your Synology and dir855 settings how you got this to work?
Everything worked with my (broken) netgear. I have not been able to get it working with the dir855
DIR855 is an A3 revision with 1.21.EU firmware
Synology is 2.3.1139 and I use SSL/TLS, Default port-range for passive FTP
EDIT:
Found the solution!!!
Synology:
Port#:21
Use Default Port range on Passive (Copy ports: 55536-55663)
Enable: Report external IP in PASV mode
Enable: Allow SSL/TSL connections only
DIR855
Virtual Server: FTP (defaults)
PortForwarding:Paste ports: 55536-55663
Save settings -> reboots router
I use FileZilla
ServerType FTPES
Transfermode: Active
This all seems to work...