https://forum.opnsense.org/index.php?topic=8998.0
Navigate to System -> Settings -> General and do the following changes:Under Networking, check the Prefer IPv4 over IPv6;DNS servers:103.86.96.100, Use Gateway: none;103.86.99.100, Use Gateway: none.On DNS server options, uncheck Allow DNS server list to be overridden by DHCP/PPP on WAN
Navigate to Services -> Unbound DNS -> General.Enable: check;Listen port: leave blank;Network Interfaces: All;DNSSEC: uncheck;DHCP Registration: check;DHCP Domain Override: leave blank;DHCP Static Mappings: check;IPv6 Link-local: unchecked;TXT Comment Support: leave unchecked;DNS Query Forwarding: check;Local Zone Type: Transparent;Custom options: leave blank;Outgoing Network Interfaces: NordVPN (or whatever you named your OpenVPN interface);WPAD Records: leave unchecked;