You could cancel the transaction with buyer's agreement, via the dropdown menu arrow for the item on your orders list. Then relist for the buyer to buy again.
During the relisting process you may be able to opt out of Simple Delivery and choose your own, more reliable, courier by following the advice on here:
https://community.ebay.co.uk/t5/Seller-Central/How-do-you-list-anything-bigger-than-61cm-Part-two/m-...
If you can't opt out maybe a new listing will trigger a new collection arrangement with DHL. One that works this time.