After change the Web Management Port, did you reboot the router first before you open port 80 for the server? Can you provide some screenshots to show what you've done on router page?

sb1370 wrote I know it, and I want to buy a wireless router because I think wireless routers provide better results than ADSL wireless modems. I just want to know how is Archer C2 wireless coverage...

How did you access it remotely? You can see Internet Access and public address on USB Settings---FTP Server page, right? In Help, it says "users in a private LAN can access ftp server via Public...

Did you select Get IPv6 prefix delegation? http://forum.tp-link.com/showthread.php?79890-Archer-C9-no-IPV6-Lan-Address&p=156909&viewfull=1#post156909

Powerline extenders have their own SSID in default settings. Can you detect Wi-Fi named TP-LINK... showing strong signal? To clone SSID and password from the main router, there are two ways. 1....

I have modified the post to answer your first two questions. As to the last one, I didn't use it with multiple users on one PC.