In certain instances, an iPhone may not display the available keys and here are some things to try...
1: Make sure location services are turned on!
2: Make sure location services are turned on for Safari specifically.
--> Go to Settings
--> Scroll to and select Privacy
--> Select Location Services
--> Scroll down and select Safari Website.
Make sure the "While Using the App" is checked.
Don't forget to make sure the membership level that your member is assigned to, has key access. This is found the Doors --> Schedules --> Edit Schedule --> Membership Access