Good morning, this issue could be one of two issues.
Firstly I would look to check the door seals are not damaged, and the door is making good contact all around the fridge when closed, it may be the fridge is cooling, then the cool air is escaping as the door isnt closed properly, or the seal has failed.
If there is an issue with the door seal, I can give you instructions on how to change this, its about a 10 minute job, and not very technical.
Once your happy the door seal is in good order, this only really leaves the the thermostat (which regulates the temperature).
AEG models are often difficult to replace the thermostats on as you have to trace the cable from inside the fridge compartment all the way back to the control panel, for this reason I would suggest you look to get someone out to replace the thermostat.
To give you an idea on cost your looking at about £35 for a thermostat and £70 for a full height door seal, plus about half hours labour\call out charge, which changes massivly depending on your location.
If you would like me to recommend someone to come out please provide your postcode
I don't have anyone in South Africa I can recommend I am afraid. Appreciate if you can rate the service received so I may close the case
I dont believe the engineers suggestion would be the cause of having a significant shift in temperature (5-10 degrees), this would however be more likely to be a thermostat failure.
A blockage in the coolant duct would affect cooling but it wouldnt cause such a variation.
I hope you fridge gets fixed soon.