You state there's a cable modem and such things do not supply more than one device. That is, it can but I'm going to take your word as-is.
So with a cable modem we never except in a rare condition and a lot of cost run that to a switch or hub.
What we usually do is get the cable modem working then add a router. A router has a few ports and today, WiFi. So to wire up the rest of the house we have to run Ethernet from the router to each room, or run a line from a LAN port on the router to a switch (I take it you will catch up on lingo) then from there each room gets a run to from the switch to each room.
To me there is no troubleshooting required yet. I think there is just wires in walls but no one has set up your network.

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