I'm glad people are reporting this is now fixed at Sky's/TalkTalk's end. My software engineer mind would love to know what the actual problem was!
A few messages ago Andy thanked me for posting the solution, but it was Annie who first suggested changing DNS servers.
For those people who changed their router configuration to use 1.1.1.1 or 1.0.0.1 there is no need to change it back. However, turning off Advanced security on my Eero will now, presumably, no longer protect me from "accessing sites that host malicious content or viruses, botnets, phishing sites and more". That sounds like a good feature to have, so I'm seriously considering turning that at on which, in my testing, will then ignore my custom DNS configuration and go back to using TalkTalk's DNS servers and block some unsafe sites.
For people who changed their DNS configuration on their Sky box from automatic (using DHCP) to manual (using static configuration), I would recommend changing back to automatic and DHCP. A lot of the comments here suggested people only changed the DNS setting which will have left the Sky box using a static/fixed IP address in the range that the router can use for assigning to other devices on the network that still use DHCP. Days, weeks or months into the future that address might be given to another device, resulting in two devices having the same IP address. That could cause strange problems which would be very difficult to diagnose.
