How to solve the problem with wifi network in the office?

Question

Computer networks

The bottom line is, there is a 4g modem Huawei B310s, such a modem was not issued by the provider. After the New Year, problems with the Internet began, as it turned out, through our fault, because. one of the users loaded the network with torrents. In addition to the Internet, a wi-fi network is organized through this modem; we connect via the network to the computer to which the printer is connected. Previously, there were problems only with the Internet, the network was slow, but working. Specialists from the provider came and replaced the modem, although no one asked them to do this. After changing the modem, printing problems began, it is simply impossible to send the document for printing, the window with the queue freezes and that's it, then the document is printed halfway and freezes. At that moment, I began to ping everything that was possible, a computer with a printer, a router. As a result, it turned out that periodically we have wild pings up to 2000-3000 and packet losses up to 30%, losing printer package and freezes. And this is not a constant thing, sometimes everything works well, sometimes disgustingly bad and unacceptable. I note again, there were no such problems with the old router. New specialists with a higher rank came, they began to say that we have too many devices, because of this interference (20-25 devices on average). But what is the funny fact, turning off all the devices and connecting to the router via wire, the ping is just as high, experts noted this, but bend their line that the problem is in the number of users and they cannot help. I put pressure on the fact that 1) Pings and losses even over the wire 2) There were no such problems with the old router. New specialists with a higher rank came, they began to say that we have too many devices, because of this interference (20-25 devices on average). But what is the funny fact, turning off all the devices and connecting to the router via wire, the ping is just as high, experts noted this, but bend their line that the problem is in the number of users and they cannot help. I put pressure on the fact that 1) Pings and losses even over the wire 2) There were no such problems with the old router. New specialists with a higher rank came, they began to say that we have too many devices, because of this interference (20-25 devices on average). But what is the funny fact, turning off all the devices and connecting to the router via wire, the ping is just as high, experts noted this, but bend their line that the problem is in the number of users and they cannot help. I put pressure on the fact that 1) Pings and losses even over the wire 2) There were no such problems with the old router.
If someone has come across this, suggest where to dig.

Answer 1

It looks like a ring in the net.
Or the router itself is flooding.
On wires within one segment, it should ping well if the wires and the switch (built into the router) are working.
Try to run Wireshark, what if you see something interesting/anomalous?
If you see repeated packets and a high frame rate, then by disconnecting one wire from the router, try to find the source.
If there is a managed network equipment, you need to look for a source on it, and if the network equipment can STP, be sure to configure it.
For WiFi, especially if 2.4G - 20-25 people is a lot. Consider installing additional access points.