Radiator Fan only working when the engine is turned off.

The fan is controlled by the temperature of the radiator outlet, this is logical, as if the temperature at the radiator outlet is between 95 and 110C (at part throttle), or between 85 and 95C (at full throttle), then then fan is not required, so won't run.
If the temperature at the radiator outlet is above 110C (part throttle), or above 95C (at full throttle), then then fan will run.
The speed at which the fan runs depends on the temperature difference between the engine outlet (G62), and the radiator outlet, (G83).

Thinking about your problem, where the fan does not run despite the engine temperature (G62) being high, I think that's because the thermostat is not opening.
It's a guess, but a logical one, (IMO).

Here's why: The FSI has a dual circuit cooling system, one circuit is through the cylinder head, and heater matrix, and is always open, (unless cabin HVAC is turned OFF). This cooling circuit is going to do a fairly good job in winter, with heat being lost from the coolant in the heater matrix.
So any rise in coolant temperature is going to be slow, hence your 25 - 30 mins before the gauge shows over heating, (above 110C, or 95C, if you're on full throttle), The other cooling circuit involves the engine and radiator, but there will only be flow when the thermostat is open.
I think the fan is not running, because with no circulation through the main cooling circuit, (because the thermostat is closed), the temperature at the radiator outlet will not rise much, certainly not to 110C (or 95C, if your on full throttle).
You can check the temperatures with VCDS.
Group 134, Block 4 is Engine Outlet Temperature, G62.
Group 135, Block 1 is Radiator Outlet Temperature, G83.
The text under each block will not show the correct wording, but the values will be correct.
The text under the blocks is wrong because there is no English language label file, in VCDS.
(This is being worked on by @Andrew and @Seb_Gurkyh as we speak. I've done a bit too, and just happened to have worked on the cooling system yesterday).

Mac.
 
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