Cant plug computer into cable modem

Hey everybody. Before i discuss this issue, im am going to try to try to prevent the flame war before,it happens. I work for a major cable company that provides internet service. My job exists at the PHYSICAL LAYER. I get sent out to homes to install coax in a way that it will work. I also verify the integrity of those cables. In case it isnt clear, I operate at the physical layer. So with that being the case, please dont bitch at me because i work for a major cable operator and am asking questions beyond the scope of my job. Sorry for the rant, but the last time i posted here, it was a nightmare. Just needed to get that out of the way.

Ok! I have had issues c9nnecting a computer to a cable modem at my house. I was trying to see what my speed was when bypassing my POS router. Easy enough, plug in the computer and youre online, worst case scenario a reset might be needed, right? Well typically yes, but not now. I have tried multiple computers and ethernet cables, but the computers cant connect to the modem if the modem is online. If the modem is offline however, the computers have no issue at all connecting to the modem and going into the gui, but as sooon as the modem finished synching up with our,central office, it disconnects the computer from the wan port. Plugging a router into the modem works just fine, and you can plug the computer into the router and it works just fine. But not directly to the modem. Interestingly enough, the modems are also reporting T4 timeouts, which is not the case. T4 timeouts are characteristic of signal issues, of which there are none. Typically when connected to a modem directly, the comouter will receive a public address. Whats happening now though is the computer is receiving a private address that is in the same subnet as the address used to reach the modem gui page (cant remember what that's called, internal address maybe? Anyway, its not important) and its not receiving dns servers either.

I suppose my question is, whats different about the router that it can connect without issue, but a computer will reset the modem? I tried changing the mac address of the computer to be identical to the one used by the router, thats not it.

Thanks guys! Like i said in the,beginning, and i cant stress this enough, please dont berate me for asking this question. I understand that i work for one of the most hated industries in the nation, and they should make sure i can do my job effectively. My job is at the physical layer, which is confirmed as being a nonissue. The issue has been escalated to somebody who's problem it is, but this is going to keep meup at night and I need to know.

Thanks!

Edit: forgot to mention a couple things. This issue isnt just at one location. Its been happening more and more throughout the city and seems,to have no correlation of variables between affected modems.