I had the same problem on 2 devices: unreachable, timeouts and no ping. I too had just replaced the modem but the new modem wouldn't issue a new certificate...or if there was ever a certificate with the previous modem.
I wish I knew what I specifically did...or did anything at all...but the browser eventually and finally displayed the Add the Certificate page.
In no particular order, things I did...
- Reset Modem from Cox Page
- Disabled Firewalls
- Forced Certificate Update
- Rebuilt TCP/IP Stack
- Updated NIC Drivers (Although I Knew No Updates were Available)
- Direct Wire