How to prove you've been in Beijing for more than 14 days
Two problems with this so-called solution.
1. This proves nothing. One can easily travel with a burner phone, leaving their main phone at their residence.
2. Very few Chinese people actually know what all these various "health" certificates are.
Until ALL communities and other places where there are retail shops are educated, one may as well not bother trying to enter if one has non-Chinese characteristics (skin, eyes, hair, etc.)