Hi there, @littlemarine.
Let me share insights on what happens after you move your accounts from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO).
Once you migrate from QBDT to QBO, certain info won't be brought over. This includes the beginning balance of your bank accounts. This is the reason why it's showing a zero balance in when reconciling the account.
What we can do is to enter it's actual ending balance which matches to your real time bank statement. Then, reconcile your account so that it would create a new beginning balance. That way, we can match it with your bank statement. Please see the screen shots below for your reference:

For more info on how your books move from QBO to QBDT, you can check out this article: Learn how features and data move from QuickBooks Online to QuickBooks Desktop.
Should you have more questions about QuickBooks, feel free to message again, @littlemarine. I'm always delighted to help you out. Have a good one.