Of course, I can override OS in WebRequest, same with user-agent if there is possibility that some server blocks connections from MetaTrader.
I was too fast with this answer It’s not possible to override OS or user-agent in MT4 I forgot that in WebRequest I can only override referrer from “direct” to some other.
Are you sure it doesn’t work? I tried it and it checked the server logs and was able to change the User-Agent using the code above. Just to clarify when you’re using WebRequest you are actually using wininet.dll through MT4 without calling it directly. Anyway the User-Agent was just a test and isn’t really needed. I thought I was being filtered on the User Agent but this is not the case. This is purely to do with TLS incompatibility in Windows XP. From your spreadsheet anyone connecting with User-Agent Windows NT 5.1 is using Windows XP which is where these problems start to appear.
Will post back on today’s results later tonight