Why does the WiFi network fall off?

Question

WiFi

There is the following equipment: Zyxel Keenetic Ultra with a Yota modem on board, 2 Wifi 2G and 5G networks (one at 2.4 MHz, the other at 5 MHz).
Number of clients per 5G network: 4 MAC laptops, 1 iPad, 3 iPhones and 2 Android phones = 10 devices in total.
2G network clients: 7 laptops, 4 android phones = 11 devices in total.
All this computer equipment belongs to 10 people (each has a laptop and a phone).
The problem is this: everything works fine, but there are moments (spontaneous) when someone, or everyone, falls off the Internet. I set a continuous ping from the laptop to the router - packets are lost, it happens that the ping jumps, for example 3000ms. It is precisely at these moments that everyone has trouble with the Internet.
The router tried to change. Initially, there was a Zyxel Keenetic 4G, then I took Asus RT-AC52u - packets were also lost and the Internet fell off. Now I put Zyxel Keenetic 4G into access point mode, put it on a cable, divided the WiFi 2G network into channels (3rd channel on Ultra - the main router, 11th channel on Zyxel 4G) - this does not help. All the same the network falls off.
Tell me where to dig?
If there are few clients (2-3), then the Internet works fine.
The processor load on the router is minimal 1-7%, RAM 27MB out of 256MB possible

Answer 1

You have too many clients. Usually, for normal operation, there should be no more than 8 clients per device.
Put another one or two wifi points and life will get better.

Answer 2

now we are testing the hypothesis - they turned off WiFi on the phones.