Freezer is slow to cool

I have a Kuppersbusch IKE458-4-4T fridge freezer but the freezer is not working as it should and I am hoping people on here can help me identify the problem.

The food in the freezer seems to be frozen and I haven’t noticed problems there however there is a very active thermostat and an alarm that triggers when the temperature is -11 Celsius or above. This, as I am sure you can imagine, is very irritating.

If I don’t open the freezer door, it will cool to about -16 but won’t get as low as -20. If I open and close the doors a few times – nothing excessive, just day to day use, the temperature rises and the alarm triggers shortly after closing the door again.

Clearly the freezer is working to a point, it’s just not as powerful as it was. There was ice built up at the back so I took the panel out and cleared this but it hasn’t solved the problem. It’s been a few weeks and ice has built up again at the back, though not badly. The fan appears to be working well.

Any ideas what the problem is?