Can't change platform toolset in VS2017

So I must have Visual Studio 2013 installed next to VS2017, because I need the 2013 platform toolset
for some projects that don’t work with newer versions. That is no problem because I can change the platform toolset from 2013 to 2017 and vice versa in every project settings, except for Unreal 4 projects, there I can not chose anything for the platform toolset (it isn’t shown).

How can I fix this? Where can I specify to use the 2017 toolset with Unreal projects inside Visual Studio, or do I have to specify that somewhere for the Unreal Build Tools??