The reason for setting the detector temperature higher than the column temperature is to have the best thermal control in the detector, to prevent drift and excessive noise. Since you can't get there with the possible settings, you need to insulate the line from the column/column oven to the detector VERY well, so that the column oven can take over the temperature control. Alternatively, you can attempt to do the exact opposite, and leave the tubing between the column and the detector in an active room temperature air stream to cool it down, and let the heater in the detector do its job. Don't know which will work better.