How to make a high-quality network of a small establishment?

Question

Computer networks

Good afternoon!
There is a small community facility. Room area ≈ 130 sq. m. 4 rooms, separated by walls of some kind of "limestone sand mixture."
On one side of the room (note: not in the center) there is a Netgear WRN3500L router , and the Internet is connected to the router at a speed of 40 Mbps.
In the district, 1-2 other Wi-Fi networks work at once.
All clients connect only via Wi-Fi (maximum 15-20 clients, mostly smartphones, a third of them can be with laptops).
Sometimes the network is extremely unstable, and the interface of the router does not allow you to track who and what is doing, and it’s social phobia to approach everyone and ask.
Advise how you can diagnose a bottleneck (maybe it doesn’t break walls well or someone specifically eats traffic and you need to limit it) and make sure that everyone lives well and pleasantly?

Answer 1

Look at what ranges neighboring networks operate on and transfer yours to a free frequency range.

Answer 2

Buy a normal router with normal antennas. A household router is simply not designed to connect such a number of clients. 5-10 still all right. You have a small room to bother with building a network. Ordinary Tp-link for 1000 rubles. I have 100 meters to the next house finishes glass balconies through the bar.