My home ISP does CGNAT for IPv4, but provides native IPv6. I can use IPv6 just fine to access most of my resources, except for one specific server. I can access the server over IPv4 from my home network, and either over v4 or v6 from other networks I’ve tried. But I can’t access it over IPv6 from my home network.
What could be the problem here? Where do I begin to diagnose it
You should be able to look up the last hop that responds (via ARIN or whoever the internet number agency is in your region) and see who that ISP is. Now the annoying part is some ISPs just drop ICMP at their border so it’s not s smoking gun that they are the issue.