I use very simple software for this. My firewall can use route monitoring and failover and use policy based routing. I just send all traffic to another machine with the diagnosis part. It does ping through the firewall and fetch some info from the firewall. The page itself is not pretty but say what is wrong. Enough for parents to read what error. I also send DNS traffic to a special DNS server that responds with the same static ip address - enough for the browser to continue with a HTTP GET that the firewall will send forward to my landing page. It is sad that I don’t have any more problems since I changed ISP.

Had a scenario when the page said gateway reachable but nothing more. ISP issue. DHCP lease slowly ran out. There were a fiber cut between our town and the next. Not much I could do about it. Just configured the IP static and could reach some friends through IRC in the same city so we could talk about it.