I’ve put my NT-505 in to my core via USB and enabled it in Roon and it works. The only thing I think it could be is selecting the correct drivers and ensuring it is enabled.
My core is a Synology NAS. I use my NT-505 as the audio output device for my PC, driving powered speakers and headphones. I checked the firmware on the NT-505 and it is current (1.22). Roon worked great prior to upgrading to 1.8. I submitting an inquiry directly to Roon Labs a few days again. Waiting, I hope not in vain, for their response.
You may have already done this, but does your NT-505 have the latest firmware installed? There’s an explicit mention of Roon Ready on the Teac download page.