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.