I am a volunteer network engineer working in a building housing refugees in Greece. It's an old school and is extremely densely populated (multiple families in each classroom). Many people have phones with WiFi.
There is one very good access point (Unifi) which is unproblematic, stays up and gives good coverage. I wanted to provide another access point to address a small area not already covered. I've tried four different consumer grade ones and they've all had the same problem. The signal fluctuates between full strength and nothing at all (using a WiFi analyser app, it shoots up and then disappears). Sometimes phones see it, sometimes they don't. Or you can see it and apparently connect but there is no internet access at all (you get the "device is offline" error).
The obvious answer is that the access point doesn't support enough clients but I've googled this and the symptoms of congestion are described as slowdown and latency. Could it also cause the symptoms I described?