The refrig is only 4 years old but the same make and basic design as the last refrig that lasted 13 years.
Having experienced this before I suspected the defrost heater. I opened up the freezer inside panels and found a small amount of ice but not enough to stop cooling.
When I got to the evap fan I found that it would turn if I gave it a nudge. But if it was off it would not start turning on its own.
I ordered a replacement evap fan motor and defrost heater, (I was in it that deep might as well). Plugged it in and in a few hours it was down in the 40s
24 hours and it was 0 and 36!
Thanks APP!