This is advice that gets thrown around a lot, but it's missing the important bit of how to buy a meter that's both reasonably accurate and affordable. There are countless articles online showing how widely commonly sold meters vary in their readings. What's more, paying more often does not mean you'll get a more accurate reading. Sure, there's diminish returns and past a certain point it doesn't matter, but many meters are completely unreliable.
The problem is there are a lot of effectively 'fake' CO2 monitors. They can't monitor CO2, they only monitor something else and work backwards to estimate CO2 levels.
The cheapest real ones I've found are on the order of ~$150
I tried to do this (buy an affordable priced, reasonably accurate meter) and, after probably after reading the same articles as you, reached the conclusion that there are no meters that are accurate and affordable.
So many seem to rely on automatic ‘calibration’ - which just seems to consist of “let’s assume the lowest sensor readings of the last while correlate to baseline atmospheric CO2”.
For a meter that’s never taken outside, this is obviously going to mean it’s reading too low all the time after a few ‘calibration’ cycles. And, worse, it's likely to be more wrong in less well ventilated spaces.
I have an "INKBIRD WiFi Indoor Air Quality Monitor" (INK-CO2W) and consider it useful and reliable for my purposes.
You calibrate it by setting it outside for a bit to establish the baseline and then bring it inside. The detected CO2 level noticeably tracks with the number of people in the living room in which it is located and whether outside air is being pulled in, either due to the furnace or having multiple windows open.
We do not use the app, so I can't comment on that. And my only complaint is that it has to be plugged in otherwise it runs out of power in an hour or two.
