I have never run any of the Dusk rom builds, so I cant speak from experience but have you considered a rom issue, or checked his forum to see if any one else is having this issue.
My carrier is sprint also, and I cant imagine what else the prob could be, the only thing that ever stops my telenav from working on the NOW network is the QucikGPS download.
Did you install the cab on your Device?
Have you tried all the standard hard reset, wait 30 seconds, etc. "THE FLASH DANCE" you might have to wipe your device and start from scratch.
or you can try this cab, its the Telenav version I use.