I did a factory reset of the router.
Did you also add a new password and username.
Not sure if this will help but it does with Virginmedia.
Switch off the Modem and router.Wait for a minute and switch the modem back on.Wait another 2 minutes and turn the Router back on and see if it makes any difference.
Have you tried connecting straight to the modem with an ethernet cable just to see if it works ok that way.