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...

If you has the US firmware, then you cannot update it to other non-US firmware. It's okay to use US firmware, no big differece, just some different on Wireless Channels on 5Ghz. If you can use the...

Do you guys try reset the router to factory settings?

I think you'd better submit a ticket to TP-Link support for this issue. http://www.tp-link.com/en/support-contact.html#E-mail-Support