The compressor clutch plate can wear down slightly over time and have an over spec gap between the plate and the main compressor pulley. 0.4mm - 0.6mm is usually the range so 0.6mm is the maximum gap you want to have.
The electromagnet which pulls in the clutch plate weakens with heat which fits with your symptoms of not working at low speed.
Once on the highway, there's enough cooling air movement for the electromagnet to be slightly stronger and overcome the excessive clutch gap.
When a clutch gap is correct, the normal fluctuations in electromagnet strength aren't a problem so basically, measure the clutch gap.
I fixed mine for free
https://youtu.be/zOjqjR8Mz64
Alternatively, you may just have low gas which is overcome by the compressor spinning up faster at higher engine speeds.
Measurement or data is the key here.