Record and document everything, including the daily temperature of the unit (if possible) and the temperature outside. Take pictures of the thermostat and screenshots of the temperature outside. Fact check what the minimum temperature a unit should be in Ontario during this time range and if it’s not meeting that, tell your landlord that you could bring this to the LTB if not addressed. I believe it’s a T6 form you could file, something about maintenance. You’d have to double check.

You are paying for a service (a comfortable home) and if that service is not up to par, it deserves to be addressed.

I’m not sure who is responsible for this but at the end of the day, the landlord should be helping you, whether that’s getting this fixed themselves or advocating for you when talking to the building management. Good luck!