How do you the GPS connected to the lightbar and then light bar to Titan RT? If so, the USB on the Titan RT is only for synchronization and would not be used as a comport replacement. The Com 1 and 3 are wired for both GPS and contollers. Com 2 is just wired for GPS only. There is not like a Device Manager for the Titan RT like Windows OS have, so not a really good way to test if the comports are working, except connecting directly from GPS to Titan RT.
When you go to the Setting - Guide Mate - Lightbar - tell me what settings you have selected here.