Hi,
The manual has a statement somewhere "that any kind of settings on the wheel setup might get overriden by the underlying software (GT5 here)".
Because of that, you need to visit the GT5 menu and turn all the FF (example if you turn the FF in GT5 to 1 and the FF on the wheel to 100 the wheel will use the "1" from GT5 as value) settings of etc. (just choose simulation as you already did).
In addition to that, maybe swap the decided steering wheel in the GT5 menu (i use T500 and im fine, others said DFGT also works fine).
There is a forum on 911wheel.com in addition. You can find all answers there (i searched and found all solutions for any kind of issues there too).
Believe me, that i had issues at the start and thought that the wheel was broken, in the end it turned out, that ALL of them were setup issues.
(like the screeching of the wheel with simulation and max FF and Max steering degree. its the FF motor working at max. it vanishes when you turn steering degree to e.g. 180 and or lower FF value)
Best