The trick is the WD-40 spray. It's a spray that can be bought at any hardware store worldwide. It's world famous. Just spray a little bit in the side of the mouse where the scroller uses a small wheel which is attached to the circuit.
I found many answers saying to use WD-40… at first I was very apprehensive, but it worked like magic.
Spray it, leave it upside down for 10 minutes and then test the scroll with an open top before closing it permanently. Amazing liquid.