I fell back to installing the IPA using iTunes 12.6.3 (iTunes 12.7 removed the ability to do this) and all I get from my phone is “Unable to install”. I think this is a separate issue but it’s hard to tell, but it seems to get further than using UE to launch.
What could be causing the second issue? Is it settings? I noticed a while ago that despite having an iPhone 7 running iOS 10 (back then) that I still had to fall back to legacy build settings in order for it to work in the past.