If I connect the router directly to the computer, then internet
connection works fine, and I can see the modem management page
(192.168.100.1).
In my experience, all cable modems from all cable ISPs are tied to a computer/devices MAC address when initially setup and then need to be reset to allow any new MAC addresses to connect to that modem.
Which means, when you state you can connect the cable modem directly to the computer and you can connect it’s because the Motorola/Arris SB6121 modem is tied to that MAC address.
So what you need to do is connect to the cable modem and reset it and disconnect the Ethernet cable right away from your system the second you do that. And then when the modem fully resets, go and connect the modem to your Linksys WRT54G router.
According to what I know about the Motorola/Arris SB6121, you can do this by going to 192.168.100.1 and clicking the “Configuration” option. The direct URL to this page should be:
http://192.168.100.1/cmConfig.htm
Near the bottom of that configuration page should be a button that reads “Reset All Defaults” with the message:
Resetting the cable modem to its factory default configuration will
remove all stored parameters learned by the cable modem during prior
initializations. The process to get back online from a factory default
condition could take from 5 to 30 minutes. Please reference the cable
modem User Guide for details on the power up sequence.
This is what you want it to do; you want to “remove all stored parameters learned by the cable modem” to allow a new MAC address to be associated with the Motorola/Arris SB6121 modem.
So click that “Reset All Defaults” button, and then once you have confirmation that the reset process is happening, disconnect the Ethernet cable from that modem to your computer and just let the modem recycle and get set up again. Could take a few minutes; just watch the lights.
Then when the lights are back to what you know as normal, connect the Ethernet cable from the Motorola/Arris SB6121 modem to the WAN port on Linksys WRT54G router and you should be good to go.
All that said, some cable ISP providers require you contact them to associate a new MAC address with a modem. Sometimes it’s as simple as logging into your cable ISP provider’s website to manage your account and adjusting something there. Or calling up the cable ISP provider directly to get things done.
But when all is said and done, this is a MAC address association problem. And once that is resolved and your Linksys WRT54G router is recognized as the valid/new MAC address associated with the Motorola/Arris SB6121 modem you are good to go.
Best Answer
It means the packets, coming to your router from provider are accumulated, this hapens due to low signal level of your ISP. either you can replace your wireless router in the top most position of your roof, or you should ask your Internet provider to change the frequencylevel ...