Do you have a modem? Make sure you have internet access when the computer connects to the modem directly. And it's better for you to call TP-Link support to setup the router.

Hi, you may refer to the instruction here: http://www.tp-link.com/us/faq-709.html

I think there is no accurate number for it, it also depends on the client devices. If all the devices are streaming videos, then less users can connect to wifi simultaneously, if they are just...

Did you try a factory reset? And change the Wireless Channel to a clean one on your router.

You'd better to send an Email to TP-Link support to check the issue. http://www.tp-link.com/us/support/contact-technical-support#E-mail-Support

Maybe try a different cable between the modem and the router. And you also can connect your computer to your modem directly by an Ethernet cable, setup PPPoE connection on your computer, then see if...