I would send one more message stating politely if you do not hear within 24 hours of a time and date for collection you will deem the item to be unwanted and will be cancelling the order.
If there is still no communication when the time lapses, click on the item in Orders >> Awaiting Postage / collection. Top left click on View Order details. Far right from More Actions drop down menu slect cancel this order, selecting the reason Buyer requests to Cancel, this refunds them if they have paid.
If they haven't paid then cancel with the reason ... Buyer Has Not paid. Both will refund your seller fees, and your seller performance is not affected.
The above is when using the Seller Hub on a laptop, sorry I do not use the app.
Always put non payers, those causing issues, on your blocked buyers list so they do not return.
Block the buyer here:
https://offer.ebay.co.uk/ws/ebayISAPI.dll?BuyerBlockPreferences