You can install IPA file using iTunes 12.x onto device using below steps :
- Drag-and-drop IPA file into ‘Apps’ tab of iTunes
- Connect your device
- Select your device on iTunes
- Select ‘Apps’ tab
- Search app that you want to install
- Click on ‘Install’ button. This will change to ‘Will Install’
- Click on ‘Apply’ button on right corner. This will initiate process of app installation. You can see status on top of iTunes as well as app on device.