The fan switch in the thermostat housing is responsible for running the fan when the engine is running. Follow the lower radiator hose towards the back, underneath the intake duct. Unplug the 'sensor' in the thermostat housing, and jumper between the 2 wires with a paperclip. Key on, that should run the fan. If it DOES, then everything is OK except that fan switch that you just unplugged. Get a new one.

If that doesn't run the fan, come back & we'll go on from there.

The sending unit for the dashboard temperature gauge doesn't have anything to do with switching the radiator fan.

ps... The other fan switch at the upper radiator hose, that one's responsible for running the fan after the engine is turned off. That one seems to be working OK.