I checked my freezer temp., when it was somehow empty the thermometer indicated range between -20 to -4C (depend on opening and closing the door) and when I put stuff inside (50% full) indicated -6 to -10C, I have located the thermometer in the freezer, my fridge is a 12ft regular refrigerator and freezer with separate door, the refrigerator works well, my concern is if the thermostat or timer works? since I could not reach -18C when there are stuff in the freezer
-18C (0F) is ideal, but freezer temps fluctuate, usually with -18C at the low end of that fluctuation. It sounds to me that your freezer is fine. Your food will be fine while you scope out and buy a thermometer that you can move to different parts of your freezer, like this:
Note from that picture that the "safe" zone includes a range just below freezing. The ideal temp of -18C is about quality, not safety.
One like this takes a bit of installation, but it monitors the temperature of your freezer constantly, without you having to open the door to read it: $25 Refrigerator/Freezer Monitor
For more information from a very reliable (if very conservative) source, see what the FDA has to say.