Looking in the service manual and going off past models the lower the setting the colder it is. So number 1 is the coldest and 6 the warmest. That how most fridges work it and the service manual shows it as that.
But setting it to 3 is about the correct setting.
But if the roof of the freezer is all iced up then it may be a defrost issue in there. As the freezer has a defrost heater and timer in there.
Try this if you can. Open the freezer and at the top you will see a vent.
This is clipped on so you need 2 flat screwdriver and insert them into the clips and prise the vent off.
From there you will see the evaporator. Wrapped around there is a thin silver tube going around the evaporator. This is the defrost heater.
Now go round the back and to the left of the compressor is a white plastic box. In there is the defrost timer.
You will see a round hole in the box.
Poke a flat screwdriver in there and turn the timer till it clicks and the compressor stops running.
Now the defrost heater should come on so give it a few minutes then go into the fridge and feel the heater wrapped around the evaporator but be carefull as they can get quite hot.
If its getting hot then its possibley a faulty defrost timer here.
But if it doesn't get hot at all then the defrost heater will be faulty.
What you can also try is a full defrost of the appliance. Turn it off for 24 hours and leave the freezer door open and this will defrost the freezer evaporator. Then retry it and see what happens from there.
As the problem you are having with the fridge section could be due to the freezer not defrosting itself. So if this is the case then after the defrost it should be back to normal.
But check the defrost heater by the above and see what you find if you can.