You can reach the modem-mode Superhub configuration pages at
http://192.168.100.1
In my experience, the issue that you are experiencing is most commonly caused by a failure to restart the cable modem / Superhub after a change in the MAC address of the connected equipment.
Try connecting your network as before with the Superhub in cable modem mode attached via its LAN port 1 (nearest the co-axial cable connector) to the WAN port of the WNDR3700. Ensure the WNDR3700 is up and set to acquire a WAN IP address automatically, via DHCP.
Now try power cycling the Superhub. All being well, when the Superhub comes back up, the WNDR3700 should report a successful WAN connection through DHCP.
Hope this helps.
Phil.
---------- Post added at 12:30 ---------- Previous post was at 12:23 ----------
NB.
I have no experience with the WNDR3700. It may be reluctant to attempt to re-acquire a WAN IP address when the Superhub comes back up. If this is the case, either force it to renew via the configuration pages (if possible), or simply power-cycle it.
Phil.