germany
This may seem familiar, I have quoted it before:
Quoting § 535 BGB:
§ 535 Inhalt und Hauptpflichten des Mietvertrags
(1) Durch den Mietvertrag wird der Vermieter verpflichtet, dem Mieter den Gebrauch der Mietsache während der Mietzeit zu gewähren. Der Vermieter hat die Mietsache dem Mieter in einem zum vertragsgemäßen Gebrauch geeigneten Zustand zu überlassen und sie während der Mietzeit in diesem Zustand zu erhalten.
Translation:
§ Section 535 Content and main obligations of the tenancy agreement
(1) The rental agreement obliges the landlord to grant the tenant the use of the rented property during the rental period. The landlord must leave the rented property to the tenant in a condition suitable for use in accordance with the contract and maintain it in this condition during the rental period.
The landlord is responsible for the proper functioning of the Thermostat just as much as for the rest of the heating.
If it doesn't work and that is the reason your apartment is falling below the said temperature for multiple days in a row, you can lower the rent.
Please note that to lower the rent, it must be broken in a way that makes it impossible for you to heat up the rooms using the heating system. So for example if you could heat your rooms to 20° if you set it to 25°, that is still technically broken and surely inconvenient, but not a reason to reduce rent.
When I went to escalate with the landlord, she told my thermometer is giving wrong values.
That seems borderline stupid. The minimum temperature your landlord has to guarantee is 18°C, your heating should easily be able to give you 23-25°C if you put it that way. No thermometer is off or could be read wrong by 7°C. A thermometer that wrong could not differentiate between a life threatening hypothermia and life threatening fever.
I suggest you ask them over and offer them the ability to bring their own thermometer if they don't believe you.
Thermostats are nasty and sometimes not easy to handle. Especially elderly people often have problems adjusting them correctly since they became all touchscreen technological marvels, when back in the day they were a box with a single hardware knob. Last time, the service technician did not set the timezone correctly, and my neighbor had to wake up to their freezing nighttime settings every morning until I fixed it. How do you explain UTC and timezones to someone who wants to turn on their heating that is definetely not travelling across timezones any time soon?
If your themostat doesn't work, ask your landlord to call a service technician if they cannot fix the problem themselves. That is their obligation.