How about speedtest results? You may make a comparison. With or without the router, the speed was tested when only one PC is connected, right? What's the hardware version of your C7? Firmware is up...

Did you try to restart it and try different computers? Ensure telnet is enabled on your PC(Start---Control Panel---Programs and Features---Turn Windows Features on or off---Select telnet client). Try...

Port 80 is the web management port which cannot be changed. Can you use a different external port like 8081 corresponding to internal port 80?

What's the hardware version and firmware version of your TD-W8951ND? Follow the guide below to upgrade the correct firmware to see if you still get an error... http://www.tp-link.com/en/faq-107.html

The powerline rate is very slow accroding to your description. Did you check it via utility? If it is true, since the powerline rate is mainly decided by the quality of power line environment, you...

The screen shows TP-LINK all the time? You may take it back to the shop and test if it is faulty or not.