Since it is a used router, it is possible there are setting in there that does not agree with your internet connection. The reset described above will clear it out and reset it back to factory default. Or possibly the firmware on the router needs to be updated.
Maybe i missed it when I skimmed through these posts.
WHo do you have for your ISP?
What kind of connection? (DHCP, static, PPPoE)
If it is DHCP such as most Cable providers, the DHCP is the default setting for...