It is time to get a replacement ...if it is in warranty period.

change the wireless channel of your main router to 6 or 11 or 1.

No, i do not need it. It is not very useful for me. I have a lot of wireless devices.

Check the wireless card IP settings on your win8 PC, is it obtained by DHCP ? What is the IP address ?

Generally, we can clone wan mac address or change lan IP address or set pppoe to let the router with modem. In your case, quite possible that this router cannot support this kind of vpn...

I find a TP-link user guide about FTP function: http://www.tp-link.com/app/usb/How%20to%20Use%20the%20Remote%20Storage%20Sharing.pdf If you cannot open the FTP server, try third party FTP client...