Hi @Sylvia981
I would buy a removable key safe and attach it to a sturdy structure nearby. Quality key safes aren't cheap, but then you have one for the future too!
When the guest arrives they will contact you to get in - you can then supply the code. You may have to give them instructions to take the safe inside with them, because it could be stolen if left open (or tampered with).
I hope everything is OK - it's possible that the guest isn't ignoring you, but that something may have happened.