Multiple DNS Servers

[Wagon1052]

I run AdGuard Home on a local network & also use AdGuard DNS.

I want to use multiple DNS servers on an iPhone.

The iPhone should use the DNS servers from AdGuard Home when on WiFi.
And use the AdGuard DNS servers when on mobile networks.

Example:
[ removed - AdGuard Home IPv4 address ]
[ removed - AdGuard Home IPv6 address ]

https://removed.d.adguard-dns.com/dns-query

tls://removed.d.adguard-dns.com
quic://removed.d.adguard-dns.com

This worked on Android and Windows. But no iOS.

Is there something I'm missing?
What is the fallback DNS when not on WIFI?

Thanks.