Why is Wifi transmission speed dropping?

Question

WiFi

There is a modem router operating on dual-band technology.
The router operates on two frequencies 2.4 GHz and 5 GHz.
So the problem is that the 5GHz frequency works stably on all devices, while at a frequency of 2.4 the Internet speed constantly jumps in the intervals from 40Mb to 1.5Mb and less.
My plan is 40 Mb.
In technical support, they sent me to change the router, I changed it, but the problem remained.
Experiments with switching channels were also unsuccessful.
What could be the problem? What can jam the signal?
The speed measurement test was carried out directly in the same room with the router.

Answer 1

View the channel load with the Wi-Fi Analyzer program or similar

Answer 2

The channel is jammed by the same owners of routers. Look at which channel is the most free and hardcode it in the router. For example, I have channel 7, because 1-5 are occupied by two to three other users nearby.
From the programs I suggest inSSIDer 4