Is the temperture inside the fridge section cold enough in there?
I ran the self-diagnostic and got a code 7 for R Defrost error / R defrost heater error.
Sorry for the delay in getting back to you.
From what you say on this the evaporator will be blocked with ice inside the fridge so the temperture has dropped so thats why it wont stay at what you set it to as the temperture is higher in there.
From the error codes you got in test mode that will be right. As on these samsungs they do like to fail like this. As the fridge doesnt go into defrost mode so too much ice builds up and then this blocks the air flow and then you get this fault.
What you need to do is empty the fridge and remove the plastic rear panel in the bottom half of the fridge. Remove the vent covers to access the screws and there are more screws at the bottom of it.
From there remove this panel and behind there is the evaporator. Use a hairdryer and melt the ice there.
Then wrapped around the evaporator is the defrost heater. Follow the wires towards the top and then you need to test it with a multimeter and check for contontuity. If faulty then you need to replace the heater.
But if the heater is fine then located in there are 2 sensor. they may be yellow. Test these for resistance and they should read about 7K ohms. If any of them are faulty then you need to replace them.
So check the heater and the 2 sensors in there and see what you find . As one of them may be faulty thats casuing the issue here.