i currently have this problem also. while i dont know why the fans dont auto turn on (and any help here would be MOST appreciated!), heres some tips to get by.
you can turn the fans on manually by unplugging the respective sensors.. for the engine bay fan, its usually near the back end of the exhaust cam cover on a little bracket, and for the radiator fan, its down the right hand side about 3/4 down. unplugging these will trick the computer into receiving no signal, and default to fans on.
also, the aw11 coolant system has a bunch of air bubble bleed valves. one is on the thermostat housing, one on the frunk back wall (heater core pipes), and one on the radiator, top right. opening these will let out any trapped air. just top up again after the engine is cool.
a combination of fans on manually and radiator bled usually brings temps back to normal. quite often there is a large pocket of air trapped at the top of the radiator.
as for why the air bubble is forming, it could either be cracked coolant pipes, radiators, a blown head gasket, or even a cracked cylinder head. in my case, i suspect the latter two. have your shop check these also, especially if you notice white foam at the fill cap
my 4agte swap cant come soon enough..