Each domain controller should have own static ip address, loopback (127.0.0.1) and optionally another domain controller's static ip address listed for DNS. DHCP server should hand out both domain controllers addresses for DNS. Also check the required ports are flowing between networks.
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/config-firewall-for-ad-domains-and-trusts#windows-server-2008-and-later-versions
https://www.microsoft.com/en-us/download/details.aspx?id=24009
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