The default Wi-Fi of powerline extender is called TP-LINK... Did you try the 8-digit password printed on the bottom of device itself? If powerline lights are on on both units, it means that they have...

What's the model number of the adapter connected to your main router? Try to restart powerline adapters and check the rate via utility.

If you reset it to factory defaults, you should connect to Wi-Fi using the 8-digit password printed on the bottom of WPA271. The utility for Mac can be downloaded here....

You want to ignore ping from WAN port, right? Try to add a rule under firewall. http://www.tp-link.com/en/faq-467.html

You'd better submit all your suggestions here directly. :D http://ticket.tp-link.com/index.php?/Tickets/Submit

Did you try to plug the USB modem directly to your PC and check if internet works? On router page, WAN IP address, default gateway, DNS are obtained automatically, right? You tried primary DNS...