The transaction or cheque numbers are displayed in the REF NO. column of the Reconciliation page. If you're unable to see it you can add this column by clicking the small gear icon above the DEPOSIT column. Then, you can select Ref no.
Unexpected behavior can also be caused by browser-related issues. We can verify that by logging in to QuickBooks Online using a private browser. Reconcile the account and see if the REF NO. column is now displaying the cheque numbers. If it works as expected, you can switch back to the regular browser and clear the cache. Other supported browsers can also help you isolate these kinds of issues.
Feel free to check out these pages that will walk you through reconciling your accounts: