To resolve the crashing issue of the DigiD app on your iOS 18.2.1 iPhone, follow these actionable steps:

1. Force Close the App
- Begin by force closing the DigiD app. To do this, swipe up from the bottom of the screen and pause slightly in the middle (iPhone X and later). Find the DigiD app, swipe it up to close it completely, and then restart the app.

2. Restart Your iPhone
- Restarting your device can often clear temporary glitches. To restart, press and hold the Side button and either Volume button until you see the power off slider. Slide it to turn off, and then turn your device back on after a few seconds - learn more.

3. Check for Software Updates
- Ensure that your iPhone's iOS is the latest version as updates can contain critical bug fixes. Go to Settings > General > Software Update. If an update is available, download and install it.

4. Clear App Cache (if applicable)
- Navigate to Settings > DigiD (or similar). If the option is available, clear any cache for the app. Note that this option might not be explicitly stated in the settings for all apps.

5. Reinstall the DigiD App
- If the problem persists, uninstall the DigiD app. Long press its icon until the context menu appears, select 'Remove App', and then delete it. Restart your iPhone again and reinstall the app from the App Store - learn more.

6. Reset All Settings
- If none of the above steps work, consider resetting all settings. Go to Settings > General > Reset > Reset All Settings. This won’t delete your data but will reset system settings like Wi-Fi passwords and wallpaper.

7. Contact DigiD Support
- If the app continues to crash after following these steps, it may be beneficial to contact DigiD support for further assistance. They may have updates or specific instructions for troubleshooting the app - learn more.

Following these steps should help in resolving the crashing issue of your DigiD app on your iOS device.