Trying to update my Raspbian for the last couple of days but always keep getting this error. Checked the Pihole query log too, to see if it is blocking the connection but that wasn't the case either. What seems to be the problem here?
As the domain name has been resolved to ip your dns is fine. Are you able to ping the server? See if you have any blocklist(firewall based) that might be doing something.
I fixed this today, the raspbian india mirror is down, you need to manually set the mirror to anything but the india one. Also not all are up to date, I think I picked one of the us or uk universities