@Mini_paradis you can first check if your PC’s LAN connection is negotiated at 100 Mbps instead of 1 Gbps, you can see this in the adapter status in Windows by right clicking the network icon, opening Network & Internet settings, going to Change adapter options, right click your Ethernet and check Status. If it shows 100 Mbps then the limitation is on the Ethernet link and not the internet plan.

Since you are using a Cat 6a cable try re-seating or replacing it, sometimes even one damaged pin can cause the speed to drop to 100 Mbps. You can also test the same cable on another device to confirm it is not the cable itself. Make sure the connectors click in properly on both the Deco and the PC.

If the link still negotiates at 100 Mbps after that then it might be related to the NIC settings or driver on the PC, updating the driver can sometimes fix it