I have been having issues for weeks since replacing my Asus router with the BT Business Hub 3 router. My wireless devices would have issues connecting and even the wired devices would have issues loading webpages. After an online chat with BT it appears to be an issue with UPnP. To resolve it simply switch off UPnP (Advanced Settings>Settings>Port Forwarding>UPnP) then apply. All devices connected to the router via wireless or wired must also be rebooted.
So far it seem to have done the trick. If the problem persists I am going to try and blag a Business Hub 5 with modem built in 🙂