I am unable to get internet connectivity on my iMac using an ethernet connection.

Make a New Location, Using network locations in Mac OS X ...

System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.

Sys Prefs>Network>highlight the Interface you're using>Advanced>TCP/IP>Configure IPv4: Using DHCP.

System Preferences>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

You are apparently connected to the internet. You are using a wired connection, so that is good. Xfinity's modems sometimes get a bit wonky. I suggest that you first unplug the modem from power for 10 seconds, then plug it back in and allow it to reset. This can take a few minutes. When the light on the top goes out, the reset is done. That may clear your problem. Also, occasionally the Infinity internet itself fails, but that is usually fixed shortly. Unfortunately, it is no longer possible to contact them by phone, unless you know a number that works. Your best bet is to use another device to access their web site where you can initiate tests on the modem and subsequently chat via the internet with one of their technicians. Before you do, you will need to have access to your account number and the modem's serial number, located on the unit. They have special test and activation codes they use. Then if the problem is not resolved, they will send out a technician to change the modem - or you could take to a store. Good luck with it.