Hello there,

Thank you for keeping us updated about the result of your troubleshooting. Allow me to chime in and help you get the support you need in QuickBooks Online.

Since you've already performed all the necessary troubleshooting steps, I recommend contacting our QuickBooks Support Team. They'll need to take over and assist you further with fixing the issue involving the return.

Here's how:

1. Click the Help option in the upper-right corner.
2. Go to the Search tab.
3. Click Contact Us.
4. Share some details about your concern.
5. Hit Continue.
6. Select Chat with us.

Also, you can access your MTD settings directly in the Account and settings section of your QuickBooks. For additional reference about it, as well as how to troubleshoot connection issues, check out this link: Set up MTD for VAT in QuickBooks Online and connect to HMRC.

Let us know if you still have more questions about tracking taxes and submitting VAT returns by leaving a reply. We'll be sure to get back to you..Take care, and have a great rest of the day.