No... C7 cannot work with a USB modem as it is not a 3G/4G router...

Get the beta firmware to solve such a problem from TP-LINK support team. http://ticket.tp-link.com/index.php?/Tickets/Submit

First, to remember or forget a network profile depends on a client itself instead of a router. Normally, if we delete the old profile and don't reconnect, it won't show up on profile list. Second,...

Try to access the drive following the guide below. http://www.tp-link.com/en/faq-420.html

What's the firmware version? Is it up to date? Provide screenshots of your configuration.

Read the post here and it may help. http://forum.tp-link.com/showthread.php?81255-Archer-C7-Malaysia-Request-Pressure-for-firmware-update-to-support-Tether-App&p=161980&viewfull=1#post161980