Too vague. Can you access the router's internal IP (192.168.100.1) when connected through the Linksys? If so, then it sounds to me like a MAC issue, your device is being blocked on the WAN. If you can't even see the modem, then there's something else amiss.
Do you have the opportunity to try your Linksys router in somebody else's network? if it works there then there's definitely a local problem at your end. I was going to recommend spoofing the router's MAC but then I realised that I run DD-WRT on my WRT54GL and the stock firmware can't spoof MACs as far as I remember (I never used it on my own 54GL!

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