To add a Windows Server 2025 domain controller (DC) to a Windows Server 2012 R2 functional level domain, you must first upgrade all existing domain controllers to at least Windows Server
2016 [or remove DC1 - B], then raise the domain and forest functional levels to Windows Server
2016 [E], and finally, add the Server 2025 DC by installing the AD DS role and promoting it.
Reference:
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels