Need some help.

After the mechanic changed the transmission on my car the temperature gauge stopped working ( most likely the transmission has nothing to do with the temperature gauge but I would like to tell the whole story).

I checked the temperature gauge it is good as a unit ( i connected a completely different temperature sensor and it works) but does not show temperature with regular temp. senson in car

I thought it is broken temp sensor so i purchased a new one but it is still the same gauge does not show temperature.

Any help appreciated