:) Noted on TP-LINK Website: Some official firmware of TP-LINK products can be replaced by the third party firmware such as DD-WRT. TP-LINK does not provide technical support and does not guarantee...

Read what HiSpeed suggested here. http://forum.tp-link.com/showthread.php?75068-Wifi-with-Internet-connect-but-not-the-Router

Where did you download the firmware? Why I can only see one firmware for WPA510U V2 and it is quite old?

What's speed if your PC is connected to new modem directly?

Either some error may occur to stop upgrading or your device will be damaged after upgrade.

Normally, if we can ping it, we should be able to login its web interface. I'm not sure what's wrong...:(