so you have a modem already? if you do, then you should setup this TD-W8980 as wireless router/EWAN. here is the instruction for you. http://uk.tp-link.com/faq-543.html

it seems that when we go to 11 ac mode, n devices won't connect well.

i think it is not about dual-band or tri-band. If your mobile phone supports dual band,then that is enough.

A wireless router is the access point (where multiple computers can connect to the network), and the wireless adapter is the network card (NIC) that allows an individual computer to connect to the...

so this adapter is not showing under device manager like this? http://www.tp-link.com/us/faq-920.html if it is there, follow the steps to install the driver manually. If no, try another usb port....

reset it to factory default settings with power on, then all the settings you have done will be removed from this device.