I had the same problem this morning.
Could access Google UK, BBC.
Could not access my own web site, the host, speedtes.net, pingtest.net.
This site very slow to load but speed test on here gave my normal speed.
Could get TT emails but obviously not from my host.
Rebooted the router and it has been OK all day.
Using TT DNS, did not check before reboot, DNS now 188.8.131.52