I appreciate your effort in trying all the solutions to fix the issue when changing the payment terms, @TRHBSI. I'll provide what to do below to get your QuickBooks back on track.
To investigate the setting setup issue on your invoice, I recommend contacting our Live Support Team to review and walk you through any steps and ensure your invoice settings are correctly setup.
Here's how:
- Go to the Menu on the upper right corner of your screen, and navigate to the Help icon.

- Select Articles and support.

- Under the Search bar, select Contact Us.

Moreover, here's an article that shows our support hours for QuickBooks Online and Desktop. While this article doesn't specifically mention the QuickBooks app, you can still use it to see when we're available to help you: Get help with QuickBooks products and services.
In the meantime, you can manually edit the due date for each new invoice.
Additionally, this material guides you on how to use the QuickBooks app properly to receive an invoice payment from your mobile device: Receive an invoice payment using the mobile app.
If you have queries about invoices on your QuickBooks App, feel encouraged to reply to this thread here in the Community forum. I'm here to help you.