Here's the TIU instructions. Go to page 98 for conventional operation wiring, page 113 for conventional locomotive operation: https://mthtrains.com/sites/de...ction/20as14017i.pdf
There is a 99% chance the reason you are having problems is due to incorrect wiring or incorrect TIU set up. Make sure that the DCS signal is turned on on variable 1 &2. Before I converted a few engines in my fleet to command control I followed the wiring in the TIU instructions for conventional/command operation and had no issues.