How to set up a Wi-Fi router from a regular ADSL+Wi-Fi modem. You need to go to the modem and turn off DHCP and something else can change, how is this even possible?

Question

Iron

• I have one router and a cable comes out of it to a regular ADSL modem with an antenna, which distributes the Internet further over the air, the modem has not been connected to the telephone line for a long time, everything has been working perfectly for a long time, but the DSL light is blinking looking for a telephone line that has not been there for a long time, but she doesn't bother me.
  
• The modem works like a regular Wi-Fi router, that is, it distributes further the Internet that comes to it in the very first port on the left.
  

I'm trying to do the same trick with a friend, and the "entire" Internet generally disappears wherever possible in the house. How can it be that everything works for me, but not for him? Maybe he has lower karma? I need to fix something in the settings of the modem itself, maybe by "going" to it?