To resolve the issue of the Demus application not starting and remaining stuck on a loading circle, follow these step-by-step instructions:

1. Force Close the Application:
- Double-click the home button or swipe up from the bottom of the screen to view all open applications.
- Find the Demus app and swipe it up to close it.

2. Lock and Unlock Your Device:
- Lock your iPhone by pressing the side button.
- Wait for a moment, then unlock your device again. This can sometimes resume the app loading process.

3. Check for App Updates:
- Open the App Store on your iPhone.
- Tap on your profile icon at the top right corner.
- Scroll down to see if there are any pending updates for Demus and tap "Update" if available.

4. Restart Your iPhone:
- Press and hold the side button and either volume button until the power off slider appears.
- Drag the slider to turn off, then wait a few seconds and turn the phone back on.

5. Reinstall the App:
- Delete the Demus app by pressing and holding the app icon until it jiggles, then tap the 'X' or select "Remove App".
- Go to the App Store, search for Demus, and reinstall it.

6. Check Network Connection:
- Ensure your Wi-Fi or mobile data is working by opening a website or another app that uses the internet.
- If the connection is poor, try switching between Wi-Fi and mobile data.

7. Contact Support:
- If the app continues to have issues, consider reaching out to the Demus support team for assistance. Check their official website or customer service resources for help.

These steps should help resolve the loading issue with the Demus app. If you continue to experience problems, look into community discussions for additional tips and troubleshooting methods such as those shared on forums like Reddit. You may also find users have faced similar issues and have shared fixes that could work for you.

For further detailed assistance on similar issues, check these discussions:
- Close and relaunch app - learn more
- App updates and potential fixes - learn more
- Troubleshooting stuck loading screens - learn more.