From the information you have posted I see two possible sources of problems. They are distinctly different because you have performed both a clean install of the OS and changed router configurations. The problem could be linked to either, neither, or both of these events. There is no guarantee that the things I am going to recommend will solve the problem, but it is logical to me to start here. I have experienced and corrected these problems on my own machines (with and without help).
Please do yourself a favor. Don't try to change other things or jump ahead of me on this. Anything you change can complicate the attempted fix. If these two things don't fix it, a more advanced member of the forum will have to guide you through other troubleshooting.
On a clean install of Windows 7 on one of my machines I encountered NIC driver
problems. I had to go to the manufacturer's website on a different computer to obtain the correct drivers
to establish an Internet connection. Do you know how to identify your network adapter in Device Manager?
Go to the network adapter's website, and locate the correct driver for your network adapter and
the version of Windows 7 you have installed. Save it to a flash drive and install it from the flash drive to the computer you just performed the install on. Reboot, and try again to establish an Internet connection. Post your results.
Reply back, and I'll provide instructions.