There is a related Q&A here. http://www.tp-link.com/en/article/?faqid=690 Why internet speed is slower when devices are connected to the router than connected directly to the modem? A: To figure out...

Repeater mode on WR710N should be OK. Wireless router>>>>>><<<<<>>>><<<<

Upgrade the latest firmware first. Your version is too old... What does "transmitter" mean here? Did you refer to WDS feature or something else?

Hardware version and firmware version of your C7? If there is a newer firmware, upgrade it. How did you test the speed? Describe in details. What's the speed when connecting to C7 by wire and...

Are you connecting the modem to WAN port of WR940N? To access http://tplinklogin.net, you have to connect PC to one of LAN ports of WR940N, or detect the router's default Wi-Fi name and connect to it...

Is it still under warranty? Get it replaced maybe. Normally, if the firmware is for WDR4300 V1 indeed and you upgrade the firmware by cable connection without power cut-off, it should not cause a...