Unable to adjust Caple c126f thermostat

Hi, I have recently moved into a flat with a caple c126f freezer. The freezer is a couple of degrees above where ideally it would be. I have found a copy of the manual online and that confirms the thermostat is the control I assumed it was but I am unable to move it beyond the current setting of 1 (the max is 4). The control has a groove in it which a penny can be pushed into but I can only move it to the off position and back to 1, no further. The manual gives no clue that I should have to do anything special to adjust the thermostat. The freezer was off when I moved in less than a month ago (and had been off for some time) so it is unlikely to have anything to do with frost. Any suggestions on next steps would be very helpful. This is the manual I referred to; https://www.yumpu.com/fr/document/read/55061308/kitchenaid-c-126-f-freezer-c-126-f-freezer-en-853937415000-mode-demploi

Thanks