thanks all for the replies. I need to make myself clear. I can start the hotspot on my iPhone and then connect to it using my iMac with iOS 11. That’s not the issue. When the iPhone was running iOS 10 I didn’t have to turn on the hotspot on the iPhone first. I go to my iMac and in the airport menu I can select my iPhone and then the hotspot on the phone will be started.
Apple explains how to do it in iOS 10 in this page:
https://support.apple.com/en-us/HT204681