Refrigerator is warm but freezer stays cool
three main points come to mind (in this order):


Door seal(s): even though you may think the door is closed all the way, a weak magnet or bad seal will cause this problem EVERYTIME. Try the "dollar bill" trick: place a dollar bill in the area around the door seal and do a full series of tests the full perimeter of the door. The dollar bill should be held in place by the door seal when closed and should require effort to pull it out. If it is "easy" anywhere, then the seal is poor and moisture is getting in greatly accelerating frost buildup
Air fan: While you identify the fan is working, is it working properly? The motor itself can be marginal, running poorly, or not at all, check contacts and wiring, and also confirm the bearings are firm but the blades spin freely. Also (and this is critical), ensure the supply and return ducts are not blocked. Make sure they are fully defrosted, and clean. Any food, mold, plastic bags, etc are removed and do not store products in front of or blocking the ducts
Refrigerant cycling: if there is a flow issue (pinched lines etc) or too low refrigerant, the unit can be cycling on and off or never off, either way allowing the ice buildup, but this is usually only found in old or damaged units. Try setting the freezer to its warmest acceptable level, and make sure it is cycling off vs. running 24/7


As noted in the other answer, but amplifying it a bit further, make sure that if the unit has a defrost tray that it has water in it - it is possible the ductwork is bloxked by plastic debris or mold/food. Make sure all tubing is free and clear