The originals are an embarrassment, even the $250 ones. I believe one that's for 450 euro has the issue fixed in it, but then that's too much of a price.
Does the thermo gauge needle rise at all or not ?
Radiator stays cool ?? U mean to say that the water is cool in it ? Is the water body working fine, how much have you driven this way ? I tell you, if it were a problem, you'd have a big problem on your hands by now. Just open the radiator cap, start the engine and see if the water goes up and down and rotates in the radiator.
Have the thermo sending unit and gauge checked.