Today i updated to 4.6.0 in my brume 2 and manual DNS stop working, i have my own Wireguard Server and i use adguard home in that server to manage DNS so the DNS requests are done internal ip over wireguard encriptation, this worked ok for the last 2 years but today after the updated stoped working.
The router will override the DNS request from the client devices.
If this option is choosen, the router will use custom DNS over VPN dns. In your imsage it will 10.66.66.1. But if this option is not chosen, it will use the dns in your vpn config.