EV Office Mail App configuration issue

Hello all,

The EV environment: EV 12.4.1 + Windows Server 2012 R2

Exchange environment : Exchange 2016 + Windows Server 2016

We are trying to configure EV Office Mail app for OWA. Inside the internal network all works fine before configuration for external users. Run a command from Setting up the Exchange server archiving Guide

Add-Type -AssemblyName System.Web
$Mbx = get-mailbox "mailbox"
New-App -mailbox $Mbx.LegacyExchangeDN -Url `
("http://EV_server/EnterpriseVault/OfficeMailAppManifest.aspx?LegacyMbxDn=" +
[System.Web.HttpUtility]::UrlEncode($Mbx.LegacyExchangeDN))

Then we configured web - access to archived e-mails via https://alias_EV_server/EnterpriseVault. All works fine in this case too.

But when I have been applied the the command: 

Add-Type -AssemblyName System.Web
$Mbx = get-mailbox "mailbox"
New-App -mailbox $Mbx.LegacyExchangeDN -Url `
("http://EV_server/EnterpriseVault/OfficeMailAppManifest.aspx?LegacyMbxDn=" +
[System.Web.HttpUtility]::UrlEncode($Mbx.LegacyExchangeDN) +
"&BaseURL=https://external_access_server/EnterpriseVault")

Nothing works.

What is the issue? I think it should be worked.

Please let me know where I'm wrong.

Thanks and Regards,

Marat