Positive ventilation pressure

I have temporarily bought a portable AC unit until I can get a proper split install.

It’s an iPAC-55 which has 18,000BTU.

The room I’m trying to cool is roughly 5m x 5m x 2.4m. The doors are shut.

I have installed it in the attic above the room and using standard insulated ducting, ducted the hot exhaust out the building and cold through the ceiling of the room I’m trying to cool. It’s set to the lowest temperature of 17*C, compressor is running, not variable and drawing 1400W.

The air coming through the ceiling is ~10C using an IR gun. The room starting temperature is 28C at 1.3m above floor. Doesn’t matter how long I run the unit for, the temperature won’t drop beneath 22.5C.

I’m trying to diagnose what’s going wrong here. I don’t have a return duct, I can feel cold air seeping under the door. Is it possible that the air being pushed into the room from above is simply falling to the floor then pushed out under the door from the positive pressure And that I need a return to get this working?…or is that “not how it works”.

Please let me know if I have missed any info.