You can download Visual Studio 2017 for Mac from here. You probably need to have a Visual Studio subscription (the free Visual Studio Dev Essentials is enough) as the download is actually delivered through the Visual Studio Subscriptions download site.
This is the version that you can download: Visual Studio 2017 for Mac (Release date: 13/May/2019)
Edit: Without knowing the reason why you can't update the OS version, the 2011 Mac's should support up to macOS High Sierra, which is the minimum for Visual Studio 2019 for Mac. Perhaps that would still be an option too?