Azure URL Rewrite issue with Milestone

Question

• Currently we have a Web App with URL Rewrite in Web.config.
• We're trying to redirect requests to external Milestone server hosted on a private network.
• A VPN is set up between the Azure Web App virtual network and the private one.
• ARR is already configured in applicationHost.xdt
• Azure Web App is used as reverse https proxy to Milestone

Currently the redirect rule in web.config file is

<rule name="Proxy" stopProcessing="true">
    <match url="^video/?(.*)" />  
    <action type="Rewrite" url="http://XX.XX.XX.XX:XXXX/{R:1}" />  
</rule> 

• The Connect and Login requests to the server work - data is returned as expected.
• When images from the cameras are requested the responses are empty and one in 400 requests returns a normal or broken image - all requests are returning 200 status code.
• When requesting the index.html page of the external Milestone server it takes 3-5 minutes for page loading.
• When using tcpping on the server the packages are received successfully.