I know exactly how you can add the deleted transaction back to QuickBooks Online, @UniIlo.

When a reconciled transaction is deleted, you'll have to first review the Audit Trail page, then locate and take note of the transaction detail. Once done, you can proceed in recreating it:

1. Open the Gear icon and hit Audit Log.
2. Choose the Filter drop-down and select the right user, date, and event(s).
3. Once done, hit Apply.
4. Then, find the deleted transaction.
5. When you locate the deleted item, hit View under the History column.
6. You'll see all the information needed for the transaction under the Event section.
7. Use that details to re-enter the transaction with its original date.

After that, manually reconcile the transaction by going to your account register.

1. Select the Gear icon and pick Chart of Accounts.
2. Search for the account where the transaction is allocated. Then, hit View register.
3. Choose the transaction to expand the view.
4. Go to the check column, and continuously hit the box until you see R (reconciled).
5. Then, hit Save.

Here are helpful modules you can read about handling bank reconciliation in QuickBooks Online:

• Undo or remove transactions from reconciliations in QuickBooks 
• Fix issues the first time you reconcile an account in QuickBooks Online
• Fix issues for accounts you've reconciled in the past in QuickBooks Online

The Community is always available 24/7. Fill me in if you need extra help with banking or QuickBooks in general. Your efforts are much appreciated, stay safe!