I''d update to 11.3.1 immediately and live with not being able to use the enterprise version of iTunes. That's what I did.
You can still drag and drop apps from your Mobile Applications folder and ringtones from your Tones folder into the new iTunes. Just plug in your iPhone, select the phone...
If you're using a Mac:
However, I had to stop using iTunes 12.6.3 and update to the latest version before I could sync my iPhone running 11.3. I just learned how to drag and drop my apps and ringtones into iTunes 12.7.x.