The reconciliation feature in QuickBooks Desktop (QBDT) and the Online (QBO) version are different. You'll have to manually enter the date in QBO the first time reconciling the account.
Then, the system will show you the last statement ending date in the next reconciliation. You can check out this article to know more about this: Reconcile Hub.
On the other hand, there are specific transactions that can't be converted when moving your data to QBO from QBDT. In this case, let run a report to resolve the beginning balance discrepancy.
See the Fix an opening balance to match a bank statement article for the detailed steps.
Meanwhile, some reports in QBO are slightly different and not all are available in the QBDT. The following article will provide further information: What data doesn't convert from QuickBooks Desktop to QuickBooks Online?.
I'm also linking an article about the list of available reports in QBO for future reference.
Please let me know if you have any questions. I'll be here to keep helping. Have a great day.