Unfavorable conditions can hinder our work processes, and I can see the importance of using the mobile app in QuickBooks Online (QBO), ax1653.
Since you have tried numerous troubleshooting steps without success, let me direct you to our Live support team to take a closer look at the issue. This ensures you receive the most accurate and timely assistance.
Here's how you can get in touch with our QuickBooks Online live support team:
1. Open your QuickBooks Online account.
2. On the upper right-hand corner click Help.

3. Select the Search tab, then type Contact human enter.
4.Then click Contact Us.

5.Choose either the Chat or Callback options.

For complete details regarding our support hours, please refer to this article: Get help with QuickBooks products and services.
Additionally, you can check these articles for more information on what mobile app features are supported and what devices: Compare mobile app features.
Furthermore, you can also check these articles to learn how to connect a bank account to QBO using the mobile app: Connect your bank account in mobile app for QuickBooks Online.
If you still have further queries on mobile apps, reach us here in the community support. Have a good day.