The modem you mentioned refers to C7? When the problem happens, you can still login web interface of C7? There is no WAN IP address? Did you configure it in the same as the instruction as below?...

Is there any error message when you try to upgrade but fail to do that? You'd better provide a screenshot here. Before upgrade, did you extract the file you downloaded?

What's the operating system of your PC? Did you enable WLAN AutoConfig(Win7&Vista) or Wireless Zero Configuration(WinXP). http://www.tp-link.com/en/article/?faqid=306...

Are you using google chrome? Try internet explorer.

Ross, W8980 supports NAT loopback indeed. Did you try http://88.247.135.100/ instead of http://88.247.135.100/activecollab/public/index.php in external network?

Firmware version is 141020? The client connected to repeater can ping the server successfully or not? How did you access it? How did you access interent via VPN connection? What kinds of VPN...