DNS Server and "Use gateway" in Settings -> General

[meazz1]

I have setup 2 Adguard Home DNS server in Raspberry Pi and was wondering what should go into the Use Gateway field? The drop down shows Wan DHCP6 IPV6 address and Wan DHCP IPV4 address.
And secondly, how to setup primary and secondary DNS so the secondary becomes active when the primary is down? As is, if I turn off the primary, 192.168.4.209 DNS, the LAN devices does not use the secondary DNS 192.168.4.208. Under Services DHCPv4 i have bother DNS listed.