I'm not convinced the ECU is drawing the ambient temperature from that sensor behind the grill that displays the temperature on the screen in the car. I could be wrong, but I also don't think that sensor is accurate. I suspect the ECU is using intake air temperature for the determination of conditions of when to lock out 6th gear, and maybe that has a sensor somewhere in the air intake path? In the MAF, or right after the air filter?I don't know if a grille cover will resolve the issue, as I don't believe it takes post cooled air temp into account, only ambient, intake and humidity.
The question is whether it's also using a temperature of the air after the intercooler, because if you can drive that temperature up high enough (by blocking off the intercooler or maybe blocking off the front grill) you can possibly get the car to use 6th gear in these relevant climate conditions.