This is the first time I have seen actual SCIENCE used to explain what is happening.
This Anon has suggested a THEORY and proposed an EXPERIMENT.
A similar, and even easier experiment is to measure the O2 content at various locations in the city, and again outside in the countryside away from suburban buildup. We should see a SMALL drop in O2 in the city and it should be consistent from location to location.
However, if the difference is really significant, then something is wrong. If the O2 concentration varies wildly in different areas of the city, something is wrong. If you can correlate low O2 with the path taken by 5G radio waves then you almost have a proof. Definitely enough data to set up controlled experiments.