ello Fran31, There is no way to do it monthly you can only do it by per day, you can click on the first drop down box under report period and can select for example custom date or this month ect and then click run report and it will then bring up the data for what you're asking to see. You can leave this as feedback within the main cog wheel.
You can amend the days per ageing period in the A/P ageing report for QuickBooks Online
If this is not what you are looking for the report to show?Do you need the amount of days that it is overdue as well as to be able to see the suppliers and transactions which are overdue for a specific month?
The Ageing Summary report (I think) can only be grouped in days (default = 30).
This was the case in QB Desktop too (at least the versions I used). As a workaround (& now a force of habit) I never raise an invoice in the first few days in a month* - that way, when I run the Ageing Summary at the end of each month the columns are effectively correct.
(* or the last couple of days in January)
There is an alternative that I use for statements (since QBO statements are woeful).
Transaction List by Customer / Group by Month / All Dates / with Filter A/R Paid = Unpaid
It doesn't give you the same layout but it's the only report I've found that gives you the information & allows Group by Month.