The HVAC people are experts on heating, ventilation, and air conditioning equipment. They really don't know very much about smoke detection equipment. (Duct detectors are smoke detectors inside the air ducts.)
The monitoring people are experts on calling the police, ambulance, and fire department. They can't help with anything in your building.
The fire alarm people, on the other hand, are experts on smoke detection, including duct detectors. These are the people who can help reset the duct detectors, or replace the duct detectors if they're faulty.
Often, Roof Top Units (RTUs) come from the factory with duct smoke detectors installed inside them. These duct detectors often aren't connected to the fire alarm system because they really aren't considered life-safety smoke detectors, but if in alarm, they shut down the fans in the RTUs on the roof.
First, though, you can at least try to reset the duct detectors yourself by shutting off the power to the RTUs for about 5 seconds and then turning the RTUs back on.
If that fails to start up the RTUs, or if the RTUs stop after a short while, call the fire alarm installation company.