Ethernet: Link speed

Start with these observations

1. Read the LinkSpeed value for your physical adapter.
2. Compare one cable or LAN port at a time.
3. Record each result before changing a setting.

Get-NetAdapter -Physical | Format-Table Name, Status, LinkSpeed

Continue with the next check

Official reference

Microsoft Learn: Get-NetAdapter

These starting checks use published documentation. They are not a diagnosis of your hardware.