Similar to what BernieSF said, with a little more detail from my specific scenario. I was getting ready to upgrade my laptop to High Sierra to try and solve this, but I'm glad I didn't have to.
I have been trying to solve this for some time now, so maybe my situation will apply to someone else. The problem was the version of Safari I had installed on my older Macbook Air.
Symptoms: All the appropriate settings were enabled, and when I plugged in my phone to the USB cable, it would show up in the Develop tab, and then quickly disappear.
Setup: iPhone 7 on 11.3.1 and Macbook Air(3,2) on 10.12. On the Air, Safari was version 10.1.
I read in an Apple thread that for iOS 11, I needed to have Safari 11, but I had no option to update Safari in the App Store. There was a system update available, and the update to 10.13 available, but not Safari.
Solution:
- I did the system updates (not the update to 10.13)
- After my computer rebooted, the App Store then offered me an update to Safari 11.1.1 and I installed that
Voila! It now works!