Hello,
Your firmware version on the router is quite old. I'd recommend upgrading to RUT2_R_00.07.03.1, as I can confirm, that SFTP is working in this newest version. Firmware files can be downloaded here (make sure to read the notes section). I'd highly recommend turning the option "Keep settings" off, as that can cause some migration issues when jumping between versions is so large.
If you cannot upgrade, SCP could also be tried, as it is also supported.
Please keep in mind, that root needs to be used as a username when using file transfer protocols.
Best regards,
DaumantasG