Hi Olivier,
You mentioned "I added it in hosts just incase, I tested telnet on 6001, 6002 and 6004 and it works fine...". I'd like to confirm if you can sucessfully run Outlook anywhere under Exchange server tab via https://testconnectivity.microsoft.com/. If so, can you set up a migration endpoint? If not, could you help us verifiy the connection to the Exchange organization using the PowerShell cmdlet:
1. Connect the Windows PowerShell to the Exchange Online.
2. Run "Test-MigrationServerAvailability -ExchangeOutlookAnywhere -Autodiscover -EmailAddress <email address for on-premises administrator> -Credentials $credentials".
If there is any error, please let me know.
Thanks,
Edward Qu