You need to look at the specs for the unit. -5C is reasonable, -15C is surely pushing the limit. When you use a heat pump in such a cold climate, it is common to have secondary electric strip heat, to "jump start" the heat in the room. The mini-splits do not have that. Under the best of circumstances, a heat pump does hot produce hot air, it produces air just several degrees warmer than the current room air. Left on long enough, it will raise room temp, but it may take a while.
I looked at the website, did not see your model number exactly, but the newer inverter heat pumps do reference a range down to 5ºF, which is your
-15ºC. At these low temps, you will feel just a slightly warmish breeze coming from the air handler!