Capitol wrote:

The router shows it's fully connected with good up and down speeds.
Looks like a DNS problem as it claims it can't resolve the address.

Can you ping things by IP addr? 158.152.1.65 is an old favourite. Set
your own DNS to 8.8.8.8 if you don't mind Google knowing your lookups,
or 208.67.222.222 if you don't mind Cisco knowing them ...

I guess BT are doing maintenance and haven't bothered to tell Plusnet.

BT maintenance usually happens overnight, there is usually a list of
affected exchanges on the status page, none planned between 24th and 27th.