If buyer has opened an inr, firstly have you put the 16 digit code in to the case where it says upload tracking? Also ensure it's been entered on your seller hub under dispatched items.
personally I would then through the case send a polite message stating that Royal Mail delivered the item on the date and time shown from their website and ask buyer if it has or was perhaps left in a safe place.
My business was a finalist in the ebay business awards 2023.