Since the 365 re-licensing, the calendar is no longer synchronized with my iphone

For iPhone calendar sync, the key issue is usually the account type and sync relationship, not the Microsoft 365 license itself.

1. On the iPhone, check how the account is added:
   • Go to Settings > Apps > Mail > Mail Accounts.
   • Select the account used for Outlook/Microsoft 365.
   • Make sure Calendars and Contacts are enabled.
2. If there is no Calendars option, the account was likely added as IMAP.
   • IMAP and POP can sync email only and cannot sync calendar or contacts.
   • Delete the account from the iPhone and add it again as Exchange for a Microsoft 365/Exchange account.
3. If the account is already using Exchange ActiveSync but calendar items are still missing, re-create the mobile sync relationship:
   • Remove the account from the mobile device.
   • Sign in to Outlook Web App.
   • Go to Settings > Options > Phone > Mobile Phones or Mobile Devices.
   • Select the device’s ActiveSync relationship and choose Delete.
   • Restart the iPhone.
   • Add the account back on the iPhone and test calendar sync again.
4. If Outlook add-ins on the PC create or modify calendar items, temporarily disable them, especially conference-room or meeting-related add-ins, then test again.
5. If the calendar being viewed on the iPhone is Apple Calendar, confirm the events are being saved to the Microsoft 365/Exchange calendar and not to a local iPhone calendar or another account.

Important points:

• IMAP/POP will not restore calendar sync.
• For Microsoft 365 business mailboxes on iPhone, the account should be added as Exchange.
• A corrupted ActiveSync relationship can cause calendar items to stop syncing in either direction.

• Can't sync calendar and contacts with my phone or tablet | Microsoft Support
• Calendar on mobile device isn't synced to the calendar in an Exchange Online mailbox when you use Exchange ActiveSync