The operation cannot continue because the LDAP connect bind operation failed

Recently, I had to add a Windows Server Core 2008 located in a remote site as a member DC. Both the sites were connected via VPN

I was able to add the server to the domain and made it a domain member. when I tired to promote it to Domain Controller,I got the error "The operation cannot continue because the LDAP connect/bind operation failed"

I tried all the workarounds, I could find on the internet but nothing helped.

The NSLookup command worked fine and I was able to query the primary DNS server.

I tried to telnet to the primary domain controller on port 389 and it failed.

Pathping from this member server to the primary domain controller worked well too. It displayed that the ping went from this member server straight to the primary domain controller

I switched my focus on the primary domain controller and tried pathping from the primary domain controller to this member server. this time it revealed that the ping went from the primary domain controller to the default gateway and failed.

I checked the default gateway and found that the VPN was not bound to the default gateway but a different device.

I added a static route on the primary domain controller to route traffic via the correct VPN gateway and ran the dcpromo again.

This time it went smooth and promoted the member server as a domain controller for that site.