Cannot connect to modem/router with ethernet cable wiFi is fine

Greetings frederickfromrichmondhill,

Thanks for the question. If I understand correctly, the Mac will not connect to the internet if it is connected by ethernet. I would recommend that you read these articles, they may be helpful in troubleshooting your issue.

Solutions for connecting to the Internet, setting up a small network, and troubleshooting - Apple Support

Internet connection and troubleshooting

Learn about common ways to connect to the Internet. The DNS issues section below applies to all three types.

Ethernet connection from cable modem, DSL, or LAN

These articles explain how to connect to the Internet or troubleshoot your service if you have a cable modem, digital subscriber line (DSL), or a local area network (LAN). You use your computer's Ethernet port or an AirPort base station to connect to these services.

• Information you need to connect to the Internet
• Connecting to the Internet via cable, DSL, or local area network (LAN)
• Troubleshooting a cable modem, DSL, or LAN Internet connection
• Using Ethernet to connect to a network or the Internet

Thanks for using Apple Support Communities.
Take care

You should NOT use ethernet cable and WiFi at the same time, instable. Either WiFi or ethernat cable.

What repairs were made to your iMac and who made them?

Click on System Preferences then click on Network ( Now you need to Choose how you are connecting and drag your choice to the top and it must have the green dot.) Ethernet will be the fastest. Just because you have the ability to use wireless doesn't mean you have to use it. As in my case I have my main iMac connected via Wireless and everything else is wireless downstairs.

Hope this helps and Welcome to the Discussions and Good Luck

Don Morgan

I repeat: You should NOT use ethernet cable and WiFi at the same time (on the same mac). Either WiFi or ethernet cable.

Keep this in mind especially when you are trouble shooting.