If the fan and compressor is working then this indicates temperature issue and/or an air flow problem due to heavy ice build-up inside the rear wall of the appliance.
As you have already defrosted it and the issue is still there it is likely the thermal fuse is faulty and will need replacement.
However, please try this:
Leave it unplugged and empty with door open for around 24-36 hours.
Then plug it back (expect some water on the floor) and wait for another 8 hours so it gets back to optimum temperature.
The problem should be resolved.
However, if the issue appears again then the problem is with defrost operation.
You will have to test/replace the thermal fuse and/or defrost heater element to resolve the issue.
Please do not hesitate to ask any follow up questions.
I hope you enjoyed your experience with justanswer.co.uk today.
Have a wonderful day.