Isp allotment for my service.

I have a service with Xfinity that is right around 700+ mbps, and I just upgraded my system to work in that bandwidth I got a Netgear cm1200 router/modem and a separate Netgear nighthawk wifi router. I have spent three days trying to get the modem and Nighthawk to make it onto the internet. I can get the ethernet to my computer, (finally) but have failed to get out over air no matter what I do because of IP address assignment issues. I am at a loss. Thinking of retrograding to my old system just to be done with this. I am no expert on IT, but have usually been successful in the past without insurmountable problems, using my ARRIS surfboard extended by my old (believe it or not) apple airport system. It still works fairly well as we do not game or do video production, etc. I saw in a forum that Xfinity only assigned one address and that is a conflict with a split system. Mine is split because my house cable and internet connection hub is in the basement where the Comcast cable enters the house, and having a transceiver there insures a weak signal. My old system used my MoCA setup to get decent bandwidth up out of the basement, but that is not working with the Netgear setup, as long as the ISP stuff doesn't seem working. Am I limited to IP address assignments?

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