On a DS218j I've had a similar problem. Enabling SSH is not enough.
I'll guess the names for specific options, because my UI is not in English.
There is a "Control Panel" application and in "File Services" > "FTP" scroll down do "SFTP" section, and check the "Enable SFTP service" checkbox, port number: 22.
sftp work fine, and file dates and times are set correclty (unlike when using Samba mounts).
One thing I'm still trying to resolve is mounting it with
sshfs - it works for a while and then hangs (maybe after some inactivity period). Bigger transfers with
scp seem to work fine, but a mounted subdirectory suddenly becomes empty, e.g. when opening in a file browser.