Looking through the manual for that device, its normal connectivity is directly through ethernet to attached computers etc. on the same LAN.
It's remote access / internet side is an option.
If you have followed the initial setup in the manual and found it's shared folders in Windows Explorer / Network places or whatever (not via the so-called "cloud" tools or a web browser) then you are connecting via your LAN.
Once you see the shared folder, you just copy & paste to/from that and other folders on the machine as you would between folders directly on the machine, you do not need any other software or tools.
See around page 22 in the manual - linked below.
You will only get maximum transfer speed if you are using Gigabit ethernet all the way through; WiFi is a waste of time in many places, it's often overloaded to the point of being useless.
If it's a well built unit you should get 90MB per second or so, with no other devices active on the network.
That's a gigabyte every eleven seconds, something like five hours for your 1.5TB file.
[In my personal opinion, any device sold using the word "cloud" that does not actually provide distributed and replicated storage - what could storage is about - is just an advertising scam, but anyway is should still work as a NAS device.]