All users of the provider in the house are visible. What is the network problem?

Question

Computer networks

Yesterday I dug into the tp-link modem, put dd wrt out of curiosity. Modem switched off. In general, all the PCs of our house connected to a common provider appeared on the local network. I understand this shouldn't happen.
Question. What is the problem and how to recreate it again?
And about a month ago, instead of 192,168,0,1, I registered 192,168,1,1 and ended up on someone else's router. There was no time so I didn’t have time to look at anything. By evening, this ip was no longer working.