Disable built in DNS Completely

in Edge settings, set DNS to "Use current service provider"
can be configed via group policy:

https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#dnsoverhttpsmode

set that to "Off"
The "off" mode will disable DNS-over-HTTPS.

also disable this:
https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#use-built-in-dns-client

more info:
"This policy controls which software stack is used to communicate with the DNS server: the operating system DNS client, or Microsoft Edge's built-in DNS client. This policy does not affect which DNS servers are used: if, for example, the operating system is configured to use an enterprise DNS server, that same server would be used by the built-in DNS client. It also does not control if DNS-over-HTTPS is used; Microsoft Edge always uses the built-in resolver for DNS-over-HTTPS requests. Please see the DnsOverHttpsMode policy for information on controlling DNS-over-HTTPS."

"If you enable this policy, the built-in DNS client is used, if it's available.

If you disable this policy, the built-in DNS client is only used when DNS-over-HTTPS is in use.

If you don't configure this policy, the built-in DNS client is enabled by default."

by the way, this part is a bit confusing: "However when users go home the external DNS server points that same URL to the external site page instead. "

you only have a homepage URL which is a website hosted internally, then what is the external site page?