Did you restore it to factory defaults and reconfigure it? If it won't help, get a replacement under warranty.

Operating system of the PC installed T4U? The old adapter was working with C5 or a different router? What's the firmware version of the router?

What's the IP address obtained on your laptops? Try to set a static IP on android to see if it works.

If PPPoE connection is established on PC directly, how long does it take? Your router is running in latest firmware?

Under virtual servers---port forwarding, you can open ports for devices in the network. http://www.tp-link.com/en/article/?faqid=72 http://www.tp-link.com/en/article/?faqid=785

No connection means no internet connection or you cannot connect to WDR4300's Wi-Fi successfully? What kinds of wireless clients, phones or laptops? Wired connection to WDR4300 is working? Any WAN IP...