You say not auto defrost, so likely no tray, or back panel or place for hidden ice to build up. It will likely have shelves between the draws which have the cooling parts in all exposed.
The standard temperatures are freezer -18°C fridge +4°C wine/beer area if any +12°C and the more expensive fridge freezers have solenoids to control if freezer or fridge is cooled, and normally freezer has priority so freezer cools first and only when that has reached -18°C will it start cooling the fridge. And all freezer working bits are behind a panel with a fan so air is circulated insuring whole freezer same temperature, and with de-frost cycle the fan turns off and heater switches on melding the ice which is directed to a tray on the motor so it will evaporate once back to cooling cycle.
However with cheap non auto defrost often the ratio freezer to fridge is fixed, it cools freezer to -18°C and there is no control for fridge it is simply a ratio of cooling to freezer. There is no fan to ensure even temperature, and there are condenser elements between each draw so you can't remove a draw to get Christmas turkey in. With a chest freezer opening door does not allow cool air out, but with upright it does, so although auto defrost is not really required with chest freezer, with an upright freezer it does a lot more than auto defrost, and without it operation is a little hit and miss.
The thermal insulation can fail, I had it with a fridge freezer, and when it fails ice builds up inside the insulation, so with a long defrost that ice melts and runs out, so turning back on all seems A1, however ice conducts heat better than air, so as it builds up, the unit has to work harder and harder, until it can't maintain the temperature.
My very old Hotpoint had this fault, we had a maintenance plan, but we always tried defrosting first, so engineer came out, and did not realise the real fault, until we one time did not defrost first, he opened the fridge door looked at ice, and said beyond economic repair, it then did 5 years as beer brewing fridge so can't complain.
Easy way to check is to measure power used, when new you have a annual power use sticker, if it says 365 kWh/annum then it will use 1 kWh per day, so if energy meter shows 1.2 kWh that's near enough, but at 1.5 kWh it is likely insulation has failed. Clearly the longer you test it for the better, and make sure cold before you start test, and open door as little as possible.
I did it when new and worked out annual use, it was a little over what label says, but not much, and once a year I retest, normally no change, it is only way as a non refrigeration engineer I can work out if OK or not.
When I went to get a freezer repaired the call out started at £60 plus what ever time they spend, so it's either a maintenance plan, or with cheap fridge/freezer dump it and buy new. With a fridge/freezer costing £1000 then yes worth repairing, the one I have now has an inverter drive, hardly know it's running, and when you open door it shows temperature inside, and is clearly at that cost auto defrost, the motor/inverter has 10 years warranty. But at £150 for cheap non auto defrost, it is simply buy new.
