As long as you uploaded tracking at time of sending you will be covered. As an attempted delivery covers the seller. The buyer should have had a card left through his door to advise item was at local po. The fact you have also messaged them too would hope jog them to collect! It may well be they have had a family crisis could be away on holiday now etc. there isn't much more you can do, but don't keep sending the buyer messages you have done all you can.
My business was a finalist in the ebay business awards 2023.