It definitely is how you have it setup. A DAC cannot play directly from a hard drive. It requires software player to control it. It usually requires a computer of a server with controlling software (i.e. Roon among others) and the best way to play DSD on a T+A DAC (as is also how many many many other DACs work) is through USB. If you do have Roon running, what is it running on? If the usb drive is connected to whatever is playing Roon then you can setup the external drive as a storage source. Honestly, if you have a NAS on your network and it is mapped on the device that is running Roon, i.e. the computer, then the NAS could be setup as a storage source also... so really it comes down to what is your controlling source? The DAC does not have controlling software/hardware built in. It only converts the digital signal that is coming into it into analog.
I can say for sure it is not the DAC. I play exclusively DSD512 on my T+A DAC from my Customize computer / music server.