I am trying to do a DC promo but it keeps on failing as LDAP connection fails with the on-prem server.

Question

My site-to-site tunnel is up, I can telnet ports 389, 88, 445, 135 from Azure VM to my on-prem domain controller. All FSMO roles are on one server. When I try to promote Azure VM to be a domain controller, the pre-requisites check fails by sometimes stating the LDAP connection could not be made and other times stating the credentials are invalid (which I triple checked are fine).
Tried with PowerShell as well but no luck.