In most places ventilation standards are set by building code. Point this out to your landlord, if they are not responsive notify the relevant local authority.
You literally can't have enough plants in the house to compensate for the CO2 production of its inhabitants.
You would have to place water immersed algae farms on every open space in the house and keep them illuminated with artificial light 24/7, because plants will produce CO2 at night otherwise.
Like you said, good ventilation is far more effective than any plant.
EDIT: ah, looking at a CO2 monitor, it says "Indoor air quality: Good (0 to 800ppm), Normal(800 to 1200ppm), Poor (>1200ppm) indication"