couple of question Frigidaire DM-90

Hi Cos and welcome to the forum ! I have the same refrigerator and I'm sure you're correct that the missing tray under the freezer compartment is why the lower fresh food compartment is too cold. The tray is insulated and measures 23 1/4 inches wide, 13 inches deep, and 1 1/2 inches high. The front of the tray comes up to meet the bottom of the freezer door as seen in the picture.  There's air vents in the front of the tray which is why the front of the tray sweeps upwards. With the cold control dial on number one, which is the normal setting, and the fridge cycling I'd say the cold control is working correctly. Measure the temperature inside of the freezer compartment. Mine maintains +10°F on setting number one. There is also a plastic strip behind where the tray goes. Frigidaire called it the "seasonal control". It should be up in the warmer weather and down in cooler weather. Without the tray in place the seasonal control is useless though. If you can find someone parting out a DM-90 or DM-90P they're the same. The P on the end of the model number just denotes a porcelain finish on the cabinet. In fact, your model number decodes as follows D=Deluxe, M=1950, 90=9 cubic foot capacity. Painted finish. The original Frigidaire part number for the tray is 5853820 and the front metal shield for the tray is Frigidaire part number 5859514. While these parts are no longer available from Frigidaire you might get lucky by googling the part numbers. If someone happens to have them for sale they'll pop up. That's how I found the light guard for mine.