just purchased raspad 3 and the ethernet port not working. i have changed out the raspberry pi 4
First of all, to rule out the problem of Raspberry Pi, please try to connect the network cable directly to the Ethernet port of Raspberry Pi to see if it works normally.
To check the problem of the cable, please make sure the contact head of the cable is good, and then try to plug it again.
Check the expansion board problem, replace the working cable to connect the Raspberry Pi Ethernet port and the expansion board Ethernet port, test whether it works normally.