How (can) do I go back and reconcile past months on quickbooks, one month at a time? The last reconciliation is from 8/9/2019. Thanks for th help!

You just "do it" the date of reconciliation to statement is up to you. So if your account has a monthly statement date of the 9th of month your next reconciliation will be 9/9/19 and should prepopulate when you start reconciliation. There also is no need to do month at a time if there are only a few entries. At a minimum for some accounts a single annual recon is enough