Getting ERR_ADDRESS_UNREACHABLE on some websites (including Google & Sky) for months.

[Deacster]

Appreciate a couple of people have asked a similar question, but resolution hasn't been posted. 

I get the above error on lots, but not all, websites and it includes places like Google and Sky that would be blocked by firewalls etc. Broadband seems to think they don't have valid security certificates. It happens on multiple divices when connecting to the Sky Broadband. Tried connecting by laptop by ethernet cable and it's the same. 

Interestingly, we got a wifi extender to improve reception in another part of the house and all websites work fine on the extender and are only affected when you connect to the main broadband. 

Thoughts welcome! TIA 🙂