I'm not sure what try next - so I'll just throw some things out.
Feel free to ignore them. Really :-)
The computer that cannot get online = "bad computer"
Does the "bad computer" have a tiny light by the LAN jack? If yes, is it green or orange when a LAN cable is connected? If you disconnect the LAN cable from this "bad computer", does Windows tell you that the LAN cable has been disconnected?
Maybe you have already tried this...
...but let's test one port on the modem/router & one LAN cable:
Disconnect the LAN cable from the back (or side) of the "bad computer".
(Do not disconnect this LAN cable from the modem/router's port.)
Plug the LAN cable into a computer that is getting online.
If the computer that was getting online can still get online, then we know that the modem/router port is good and that the LAN cable is good.
Can you please post a screenshot or copy/paste the ipconfig/all info from the "bad computer"? Hopefully you have a flash drive that you can use to transfer the image or text to a "good computer". Screenshots and Files - Upload and Post in Seven Forums