@ArcherC8 yes it was that, the cable.

@Bertu It was motherboard lan card fault and not the router. Sorry for making you losing time, and thank you.

Godd afternoon. I have a NAS connected to a router AX1800 24/7. However, every time I turn on my main computer, the Ethernet connection is limited to 100 Mbps instead of the expected 1000 Mbps....