Our entire family connected to the internet via wi-fi through a TP-link modem.
Recently I discovered that I could not navigate through a particular website hosting companies site because one of the pages would not open.
However when I connected either machine to the internet via 4G the page concerned loaded .
By trawling a number of forums it was suggested by people with similar issues changed the DNS server .
In the case of my mac this easily done by going to system preferences , network , advance then DNS.
I selected Google's public DNS server which has a ip of 8.8.8.8 and saved the changes and lo and behold the page now loads .
I hope this helps those struggling with this issue .
It is a pity these sort of issues and the resolutions are not added to a faults solving flow chart at BT support
If you are using a TP Link router, then its possible to change the DNS setting on that, so you would not have to change it on each device.
Bear in mind, that both BT Web Protect and BT Parental controls will not work unless if you use different DNS settings. Its most likely that one of those, which was stopping the pages loading.