I've been tracking down a confusing problem over the last couple days and I'm hoping someone here might've experienced it before.
First, the short version of the problem: My Zoom 5341J modem provides a great internet connection when hooked directly to a computer, but no internet connection when run through a router.
Location: Michigan, USA ISP: Comcast Modem Firmware: 184.108.40.206J
Now some background. I've had the same set up - an ASUS router and this 5341J modem - for several months. My only complaint is periodic (once or twice a day) 'hiccups' where the modem seems to loose but then re-establish a connection, generally lasting from 10-30 seconds.
I came home from work at about 12:15 am (I work nights) a couple days ago to find that the internet connection was down and when I looked at the modem, only the power and link lights were doing anything. I figured no problem, I'll just unplug/replug the modem and the router. When the modem came back up, all the lights were good so I plugged the router back in but after it reconnected I had no internet connection. I tried power cycling both again, I could get internet connection through the router for a brief time - probably less than a minute - before it would go down again. I ended up resetting both the modem and the router to factory defaults with the issue persisting. When I tried going straight from the modem to a computer, though, it would work - I could get to the internet just fine.
I had an old Linksys router kicking around so I figured maybe my router had died somehow so I tried this Linksys router, same problem. In the course of the testing I went back to straight plugged into the modem and no internet this time. I called Comcast, they confirmed that the signals levels seemed fine but since I was straight in to the modem and no internet they sent a tech out. He came out the next day, unplugged the main coax line from the modem and tested it, it tested fine, plugged it back into the modem and lo and behold the computer hooked straight in to it could reach the internet! The tech suggested it had to do with no unplugging/replugging the modem when I switched from a router to the computer directly - I thought I had, but apparently not. I then plugged the modem back into the old Linksys router, unplugged/replugged the modem then the router, and I had internet through the router. Satisfied, the tech left. As he left, the internet went back down through the router - I should've known, since that is what it was doing before (up for a bit after I powercycle the modem and router but then it dies out.)
I went and bought a new Belkin router, thinking surely it must be the router since a connection straight to the modem works. The Belkin has a setup utility and I followed its prompts but was unable to make a connection when plugged in to the modem and properly powercycled as per the on screen instructions. So that's 3 different routers of 3 different brands that won't work with the modem.
To add to the craziness, during my tests I was able - for a while - to get internet connection over wireless with the Linksys but not to the computer I had plugged straight into the router. I swapped back to my original ASUS and I managed to get the same thing! (ie wireless device could get internet but a computer plugged straight in to the router could not.) After powercylcing a few more times, though, even that stopped (ie I could no longer get internet over wireless, either.) I have not continued tests with the old Linksys to see if I can coax it into internet over wifi again.
So last night I tried plugging a laptop straight in to the modem and it works just fine - so it isn't a matter of it only liking that computer I had been plugging it straight in to. I then plugged it in to a different computer and used Skype on it - worked just fine. Overnight I plugged it in to the original testing computer and left it going overnight. This morning I can still navigate the internet just fine on it. It would've made me feel better if a direct connection to the modem eventually flaked out like the connection through a router does, but no, the direct connection to the modem seems to work just fine.
That brings us up to where I am now - I apparently have a 5341J modem that will not talk to a router, even though as of about 4 days ago it had been doing so without major issue for a few months. I thought maybe the modem got a firmware update that broke things, but no, it seems to be on the same old firmware from several months ago. I figured a reset to factory defaults on the modem would work, but no, the issue persists.
At this point I'll probably talk with Comast to rent a modem from them for a month to see if their modem works properly. If so then I'll know that it's something wrong with my 5341J modem. It just boggles my mind that a direct connection to the modem works but the modem to a router to a computer does not works, especially when it had been working just fine for months!
If you have any ideas or things for me to test, please share them :)
Update: This answer, and others like it on that same question, seem relevant to the discussion here, but I have (at least, to the best of my understanding) done most of these things already. Perhaps next I'll need to try hitting the reset button on the modem when hooking it up to a router and waiting longer before powering on the router.