I seem to be having trouble setting up opnsense to use a custom DNS Server. I've tried setting the server IPs under Settings | General and have unchecked allow override. Yet when I run a DNS test, it still returns my ISP DNS?
I set the below to cloudfare's DNS, yet a dnsleaktest still returns my ISP?
Would anyone know what else I may need to change or correct?
You probably did not change your browser's settings. Most modern browsers use DoT or DoH and not the system nameserver(s).
Also: These settings only determine what OpnSense itself uses for DNS. Unbound can be configured to use these servers or resolve names by itself.
And then you still have to set up your DHCP service of choice to hand out specific servers to you LAN clients, which may be any of: these DNS servers, your OpnSense (via its LAN IP) or any other servers.
The chain is: 1. browser using system or other DNS services 2. client settings (most often provided via DHCP) 3. DHCP service settings.
There is just so many forks to go wrong here.