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Have you phone the FTTP team on 08005874787 and reported the problem
is it wireless connection that is causing the problem? Did you split the networks as many devices don't like the networks combined. Then you can try different channels manually or change the wireless mode which may also help
If I read you correctly pages are loading ok over ethernet, it's just wifi that's laggy?
If you're using the Smarthub and haven't already done it then try seperating the 2.4G anf 5G networks and experiment with the modes (1, 2 & 3)
In addition if you're running windows 10 then try troubleshooting your wireless adapter (Start, Settings, Update & Security, Troubleshoot, Network Adapter) and setting the DNS on your wireless adapter to either google or OpenDns
mode 1 is the best but only if it suits your devices hence the reason for modes 2 & 3 you can try selecting the channels manually after using a wifi analyser to see where neighbours routers are connecting
Sorry I misread you. The mode setting is for wifi only, it will make no difference at all to ethernet connections.
When you say it's still laggy over ethernet is that direct to your hub or via a powerline connector and (stupid question number 1 ) how long is the ethernet cable.