Do you have access to the cable modem that would allow to make changes if necessary ??
The reason I ask is, the Ethernet port may only be capable of 100mb and if it can do 1gb it might be hard coded to 100mb and what happens is your cable modem and pc will auto negotiate to a compatible speed in this case 100mb
The only solution if the cable modem can only do 100mb is to upgrade to one that is capable of 1gb otherwise you are spinning your wheels
The cable modem is brand new Gigabyte fiber capable. The service provider’s introductory offer was 600MB and now they are upgrading it for free, only the speed because the modem is fine.