Are you using latest firmware? In future the firmware may suport 3G USB adapters? I don't think so...TP-LINK Routers which support 3G USB adapters are TL-MR...

In AP router mode, you have login its web interface via wirelss connection. Can you detect Wi-Fi name of 7210 and connect to it? What's the default gateway obtained automatically on that client? Is...

It should depend on lease time showing in the DHCP clients list.

Did you try to hard reset it? If it makes no difference, it should be faulty. Is is still under warranty?

You can refer to the table in instruction below. http://www.tp-link.com/en/article/?faqid=340 mp4 is the supported format. BUT C7 doesn't use standard DLNA, which may cause some compability issues...

Copied from http://www.tp-link.com/en/article/?faqid=690. Q:Why internet speed is slower when devices are connected to the router than connected directly to the modem? A: To figure out the problem,...