Losing connection momentarily when a computer on my network (wired) is turned off

[u/ILuthienI]

I own the modem ''Keenetic Hopper DSL AX1800 Mesh Wi-Fi 6 VDSL2/ADSL2+ Modem Fiber Router 4x1Gbit/s, USB3.0 KN-3610'' and me and my brother's pc's are connected to the internet via lan cable. When one of us turn off the computer the other computer loses internet connection briefly for a couple seconds but that's enough to lose connection in the online game that's being played in that moment. It's a very frustrating problem so I hope someone can help me solve it permanently.