How to merge two networks with two routers?

Question

Computer networks

Good afternoon. I broke my head in search of a solution to the problem with LAN in one of the companies. Bottom line: there are two Internet cables separate from each other, both cables go to separate routers. One router distributes the Internet only to one computer (real IP), the other distributes it to all other users. There are two rooms, in one there are 4 machines, in the other 5. All computers are networked with switches and one cross-panel, the wires go to wall sockets. Accordingly, 4 computers in one room are connected by one switch, 5 others - by another. Everything works separately, but as soon as you run a wire from one switch to another to connect two rooms into a network, the Internet disappears almost immediately in the room where there are 5 computers. This disables the locale. I got out of the situation as follows: in a room with four machines, I registered a router with a real IP as a gateway, the other router is stripped to another room, respectively. Everything works, but there is no common network.
The problem appeared suddenly, before that everything worked, everyone received the signal they needed, while there was a common network. I called technical support, they said that there was a heavy load on the port from our side, they asked me to check for a loop. I checked everything, replaced each switch in series, checked all the cables, each one is connected correctly and not in the same switch.
Help, please, to solve the problem, maybe someone came across? Is this a problem within the network or does the provider's ports not accept the connection of a second signal to the network?
Thank you in advance.

Answer 1

Most likely, some kind of unaccounted crossover has appeared, and when you connect the switches with another wire, you get a ring and the switches go crazy. Therefore, the network falls off. Disassemble the entire network and reassemble carefully.
Well, ideally, it would be a picture with the structure of your network to draw, otherwise it’s somehow unclear from the verbal description what you have connected where.

Answer 2

I'm sorry, I'm new at this. I hope it will be clearer now. If additional is required. information, I will try to give it.