Change to Wi-Fi. For iPhone and Outlook calendars to sync properly, the data needs to be sent to the online servers, then re-downloaded to the other device. Data syncing can be delayed when your iPhone is on a cellular connection to save data so try to connect to a Wi-Fi signal and see if that works.
Disable Airplane Mode. If you’ve had Airplane Mode turned on while watching a movie or during a flight, none of your data will sync properly, as your iPhone will be unable to connect to the relevant online servers. Check to see if Airplane Mode has been turned on, disable it if it has, connect to a cellular or Wi-Fi signal, and wait a few minutes.
Turn off your iPhone's Low Power Mode. This setting is activated when the device's battery gets low. It disables most background activity including downloads and data syncing between services.
Charging your iPhone usually disables this mode automatically but you can also manually turn it off. Go to Settings > Battery and tap the Low Power Mode toggle switch.
Greetings,
Peter