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Question: connection settings for WAG200G

hi, i had bought this product 5months back, got it connected through a shared modem from a neighbour using an ethernet cable, without doing any security settings and i could use my laptop wirelessly,
but after 2 months when i tried to do security settings ,i cannot use this gateway. did everything , called the linksys , now they say i need a DSl line to start using it, the error that i get is DNS server not responding, what settings i need to do so that works like before as i have only an ethernet cable to attach no DSL line cable.
i can use messenger wirelessly , but the webpage doesn't open.
please guide me or else this product will be worthless for me.