Servis fridge-freezer fridge compartment is not cooling - how to fix?

Hi there, we have a Servis fridge-freezer model number M7684S-A. We've just moved into the house, and the appliance came with the house, however we don't know when it was purchased/history etc. The freezer works normally however the fridge just doesn't seem to be cooling properly. We've looked on various forums, and the first advice seems to be check for air flow at openings, however we cannot find openings in the fridge for the air flow. There is a low humming noise coming from the freezer/fridge when turned on, but it doesn't sound loud enough to be a fan. We've taken the grill off the back, and there wasn't any dust there. We checked the plastic tube coming out the back of the fridge behind the grill, and that was clear/empty (no water/no ice blockages). We have taken a look at the thermostat in the fridge, but there's nothing immediately obvious there. The drainage hole in the fridge is clear. In the freezer compartment, all we could find was a removable section at the back with pipes behind it, but there didn't seem any issues there... Any guidance?? Many thanks!!