New Windows Phones Not "Mobile"?

You can add “IEMobile” as custom UserAgent.

https://blogs.windows.com/windows_phone/b/wpdev/archive/2012/10/17/getting-websites-ready-for-internet-explorer-10-on-windows-phone-8.aspx

I found the following user agent through a quick web search:
Mozilla/5.0 (compatible; MSIE 10.0; Windows Phone 8.0; Trident/6.0; ARM; Touch; IEMobile/10.0; <Manufacturer>; <Device> [;<Operator>])

Try adding any unique portion of the user agent string in the Custom User Agent field in WPtouch: IEMobile/10.0' orWindows Phone 8.0`. Be aware that adding the full string will not give the desired results.