Install app on iOS without using iTunes
Depending on your setup, you have two options.


OPTION 1 - Use iTunes 12.6.3 instead. Refer to this answer for more details!
OPTION 2 - Use Apple Configurator 2. Download and install Apple Configurator.


Since you're saying that iTunes 12.6.3 isn't working for you, than give Apple Configurator 2 a go. Apple Configurator 2 provides much more advanced features in terms of managing your iOS devices.