Most bottom mount freezer units work by diverting some of the cold freezer air up to the refrigerator through a duct that vents to the top of the refrigerator A thermostat in the refrigerator closes a flap in the duct when the refrigerator is cold enough.
It seems like the thermostat or flap is not working in your refrigerator. You can troubleshoot this by turning the refrigerator to the full warm setting and turning the freezer knob in the cold direction until you hear the refrigerator turn on. Place your hand near the vent at the top of the refrigerator, if cold air comes out that means the flap is not closing like it should.
If the flap is not closing, it might be frosted open. Pop the plastic vent off (they are usually held in with snaps) and inspect the flap. It should open when the fridge knob is turned in the cold direction and close when it is turned in the warm direction. If there is no frost jamming the flap, and the flap is not moving, the thermostat is likely defective. See the pictures below, one is flap open and the other is flap closed.