Fiber connection to my Netgear with missing Ethernet in port

You can't use the C series cable modem on a Figer ISP service line. You'll need to find a R, RAX or Orbi R series external router to connect to the Fiber ONT device. Contact the ISP for help and information regarding there support of routers on there network. 

Netgear has set up a community forum specifically for the Cable Modem products. Most of the people who watch that forum are more likely to have experience with Cable modems and know how to work it better than those of us who follow this router forum. Might be more likely to find someone who has a solution if the question is posted there:
https://community.netgear.com/t5/Cable-Modems-Routers/bd-p/home-cable-modems-routers

Thank you.