Replacing ISP with Xfinity and can't get Time Machine Airport Utility to connect through muldem to my IMAC

I recently replaced my DSL service carrier with Comcast Xfinity. I purchased a Comcast approved Netgear modem that initially worked but after I rebooted the IMAC I couldn't get the ethernet wired connection to work unless I wired directly from the muldem to the IMAC. Both the wired and Wifi were running when the tech was here. Any help would be appreciated. Is there a certain procedure or setting I should follow? I did this a few years ago and did not any problems with setting the wired and wireless network up.

Thanks,

Bob