Hello @Faruque ,
Thank you for posting here.
Based on the description, I understand you have two DCs in the domain, the IP addresses are 10.1.1.5 &4.
Do you install DNS role on both DCs?
If so, we should set the static IP address for both DCs, and we can set the Preferred DNS on the first DC as 127.0.0.1 ( or 10.1.1.4, I assume the IP address of the first DC is 10.1.1.4).
We should set the static IP address for both DCs, and we can set the Preferred DNS on the second DC as 10.1.1.4 (I assume the IP address of the second DC is 10.1.1.5).
Preferred DNS:IP address:10.1.1.4
And if you want to change one machine from WORKGROUP to the domain.
You need to set Preferred DNS as 10.1.1.4 or 10.1.1.5 on this machine first (I assume both DCs are DNS servers, that means you install DNS role on both DCs).
Then change machine from workgroup to domain.
Hope the information above is helpful.
Should you have any question or concern, please feel free to let us know.