What is the most specific likely cause of this symptom, and what is the first diagnostic step?

• A.The problem is ISP-side DNS filtering; the ISP is blocking DNS queries from VPN-connected clients as a security measure; DNSSEC encrypts query payloads end to end so resolvers cannot read them. Round-robin DNS guarantees even load because resolvers coordinate their rotations. DNS64 synthesizes A records from AAAA answers for IPv4-only clients, the reverse of its actual direction
• B.DNS resolution is failing, not network connectivity; the most likely cause is that the VPN's DNS push configuration is not providing a working DNS server address to the client, or the VPN client is not applying the pushed DNS settings. First diagnostic: run 'ipconfig /all' (Windows) or 'cat /etc/resolv.conf' (Linux) to verify the DNS server address the client is using; then test with 'nslookup google.com <dns_ip>' to verify the DNS server responds