For new users:
You may have just purchased your first Phoner data Plan.
When eSIM is ready, you can tap on "Set up eSIM" popup.
Otherwise, if you missed the popup, you can still tap on the "eSIM is ready" top banner.
After that, you should be directed to set up your eSIM via a series of Apple instructions.
For existing users:
You may have removed the eSIM profile.
If eSIM is not detected, you can tap on "eSIM is not detected, Tap here to set up" top button.
Otherwise, you can also select "Reinstall eSIM". Find it in the "Data Plans" tab → Look under last section "Learn About Data Plans" → Tap on "Reinstall eSIM".