I am assuming you are not using Apple products by any chance with Private Relay enabled.
Also have you tried different internet browsers from your default one.
Have you done any changes to your online Broadband Buddy Settings and going into the NOW Router webpage Settings and play around with the different wifi channels, splitting wifi bands, trying IPV6 over Lan enabled (default) or disabled and Ethernet untick Energy saving and Fast Ethernet instead of Gigabit.
All the above would be temporary tests just to see any noticeable improvement.
If still no joy, get in touch with the NOW Broadband Team.