It should find it automatically unless you have hard coded it.
https://vanhybrid.com/2016/01/25/force-azure-ad-connect-to-connect-to-specific-domain-controllers-only/#:~:text=Right%2Dclick%20the%20connector%20and%20choose%20Properties.&text=In%20the%20Configure%20Preferred%20DCs,OK%20to%20confirm%20the%20changes.
Verify that is not set to a specific one. If you continue to have issues, you could always set to the new DC to ensure its working for now.