You can manually trigger Azure AD Connect to perform a sync cycle. Open a Powershell on the server running Azure AD Connect, then type:
During the installation of Azure AD Connect, the registration of the Azure AD Connect Health for Sync-agent may fail. When this happens, you can manually register the agent by running this Powershell cmdlet:
Register-AzureADConnectHealthSyncAgent -AttributeFiltering $false -StagingMode $false
You need the credentials of an O365 account with Global Admin rights.