Does a purifier actually cut visible dust, or only the part you cannot see? Purifiers

Household dust is mostly textile fibre, skin, soil tracked in on shoes, and outdoor grit, and the majority of it by mass is far too heavy to stay airborne long enough for a filter to catch it. That is why the shelf keeps needing wiping while the fine particle count in the room drops. If you want less visible dust the levers are shoes off at the door, fewer fabric surfaces, a doormat that is actually big enough, and a vacuum that does not blow half of what it picks up back out.

Four months is also long enough that your filter may be the reason you cannot tell. Busy road means a lot of coarse loading on the pre-filter, and once that clogs the airflow collapses and the unit quietly does very little while sounding exactly the same. Pull it out and look at it; if the pre-filter is grey and furry, vacuum it, and if the main filter looks like a road, replace it and see if anything changes.

Mine dropped noticeably after I stopped shaking out the duvet in the bedroom and started doing it by an open window in the hall. Sounds trivial. Was not. Bedding is a fibre factory and the bedroom is where most of the surface dust in my flat originates.

Busy road, windows shut, still dusty means air is coming in somewhere you have not found. Letterbox, extractor flap, gaps under internal doors to a stairwell, trickle vents. Smoke pencil around the frames on a windy day and you will find it in ten minutes.

Whether it works and whether you can perceive it working are separate questions, and the second one is almost always no without an instrument. A twenty-pound particle counter is not lab grade but it will tell you if the unit does anything at all in your room, which is the question you are actually asking.