Freezer OK, refrig compartment warm and refrigeration system cooling condition can be diagnosed by removing freezer compartment inner rear cover. A little frost is OK but if air flow over coil is block with iced it is not defrosting properly.
First, melt ice off coil with hot air gun. Check to see that melt water is going to exterior pan at lower rear of refrig
Next check defrost heater, fuse and sensors.
At upper right where pipes to coil enter freezer compartment there are two connectors. One comes from defrost heater. The other from Sensor & fuse assembly combo.” It may help to look at online picture of combo assembly Part# 6615JB2005C.
First unplug electric defrost heater element connector. Check heater resistance with ohm meter, OK if between 30 to 100 ohms, plug back in. If open or shorted replace defrost heater.
Next to heater connector is one with 4 wires from combo sensor & fuse in clear plastic packet on refrigerant coil.
Unplug the connector and check resistance between pins to fuse. It should be less than 1 ohm. If fuse is "open" replace whole combo assembly. For a temporary fix, cut fuse wires from the packet and wire them in series a 10 amp fuse/holder. For long term fire safety a “thermal”, not “current” activated fuse must be used.
Did not have spec's for sensor resistance but mine was 12,000 ohms at room temperature. Sensors have a wide range of values based on technology and temp so there is no rule of thumb. Open or closed resistance is usually bad. Get actual spec to verify'
While there are many reasons for cold freezer and warm refrig these are some easy things to check out.