Fridge is cold after manual defrosting, but then warms

It sounds like you've already identified there's a problem with the automatic defrost function.

To dive deeper into that problem you could test whether the automatic control is working or you could test whether the defrost heating element is working. The latter is easier to test, and one might argue it is also more likely to fail.

Remove power from the appliance, expose the defrost heating element, and disconnect at least one of the two wires going into the element. Measure its resistance. I don't know anything specific about your appliance, but based on general experience, I'd expect a working defrost heater element to have a resistance under 100 ohms, and probably even less than 50 ohms. If you measure much over 100, and certainly if it measures open circuit, then the element itself has failed.