Sure, so like I said, you have a situation where a CO2 scrubber(in that case concrete) is present. If you need accurate measurement of CO2 below 400ppm and not just "it's less than 400ppm" then you need to buy a different sensor.
The problem is that it gives you percentage of CO2 in air instead of ppm, it can go all the way down to 0% though. I guess there must be a way to figure out the ppm based on that? I guess?
Here's an example of a product with this sensor already integrated:
Also I want to point out that their other sensors(SCD40 and SCD41) will actually give you a value even below 400ppm - just that their accuracy drops below that value. So it might say it's 300ppm when in reality it's 250ppm. Not sure how important it is for you.
