Can you post the model number please.
8586 158 15001
serial number 1106 150 15558
Ok so this is a side by side that's fine.
If you remove the bottom baskets in the lower half of the freezer and then look on the rear panel at the back do you see any ice or frost building up on there?
Baskets removed, but there doesn't appear to be any ice build up. I dont usually look at the panels, and all I would say is that at the back of the second shell up the surface is a bit bobbly, not smooth.
Ok usually when these start to drop temperature inside the freezer its down to an ice build up behind the rear freezer panel.
What you need to do is either empty the freezer and turn it off for 24 hours and leave the freezer door open. This will allow the ice behind there to melt.
Or if you are handy at diy you need to empty the freezer and remove the side rails in the bottom half.
Then you can remove the rear panel. From there check to see if the evaporator in there is blocked with ice. If so then you need to defrost it with a hairdryer until its fully defrosted.
Then refit the panels and retry it and see how it goes. Hopefully its just a one off but if it happens again then its a fault on the defrost side. Either the heater itself or the rare control at the back. To test the heater you need to access the evaporator unit again and wrapped around there is the heater. Test it with a multi meter and see if faulty. If it is then it needs to be replaced. But if the heater is fine then its more then likely the rear control board.
But try the defrost first as you may find its just this.
If you need anymore info just get back to me.